Gadding with Ghouls: One of Gilderoy Lockhart's books.
Galatea Merrythought: Etym: The name of Pygmalion's statue, and also of a nymph.
Galleon: A gold coin, equivalent to 17 Sickles or 493 Knuts.
Etym: Probably from the meaning "a great prize or catch", referring to the capture of Spanish galleons by English privateers.
Gambol and Japes: The joke shop in Diagon Alley.
Etym: Gambol as in to play, jape as in joke.
Ganymede (OotP ch. 14): One of the Galilean moons of Jupiter.
gargoyles (OotP ch. 17): Apparently there is a pair of them guarding the staffroom. (Technically, unless they're channeling water, they're actually grotesques.)
garlic: Rumored to be the source of the smell coming from Professor Quirrell's turban. Garlic has been credited with numerous medical powers, and the ability to ward off vampires.
Garroting Gas (OotP ch. 32): Some kind of asphyxiatory magical gas.
Gaunt, Marvolo (HBP ch. 10): Voldemort's maternal grandfather.
Etym: From Middle English, meaning "thin" but also "greedy" or "ravenous".
Gaunt (Riddle), Merope (HBP ch. 10): Voldemort's mother, briefly married to Tom Riddle senior thanks to a love potion.
Etym: See above.
Gaunt, Morfin (HBP ch. 10): Marvolo's son, Merope's sister, who spent some time in Azkaban for hexing Tom Riddle senior.
Etym: See above.
Gawain Robards: Etym: One of the knights of the Round Table, originally introduced as the model of knightly perfection, but in the later versions, he became treacherous and violent.
Geoffrey Hooper: Etym: Several different possible etymologies.
George Weasley: Etym: The patron saint of England, whose dragon-killing exploits, said to have happened in Libya, may be a transfer of the Perseus myth. The name is from Greek for "farmer".
ghosts: Dead wizards who have chosen to remain in the mortal plane, rather than move on. Hogwarts has about 20 ghosts in residence, including the Bloody Baron, the Fat Friar, the Grey Lady, Nearly Headless Nick, Moaning Myrtle, Peeves, and Professor Binns.
ghoul: A malignant spirit sometimes associated with grave-robbing; one haunts the attic of The Burrow.
giants: Huge humanoids usually believed to have existed before humans. Frequently characterized as brutish and stupid, although medieval mythology included several giants as tutelary figures.
Gibbon (HBP ch. 29): A Death Eater, who may have been one of the ones recently sprung from Azkaban,captured in the fight at Hogwarts.
Etym: From a diminutive of Gilbert.
Gideon Prewett: Etym: The name of an Israelite judge and leader. Comes from Hebrew, meaning "having only a stump (for a hand)".
Gilbert Wimple: Etym: From Old German gisil "pledge" + berhta "bright". There was a St. Gilbert of Sempringham in the early 12th century who founded an order of monks, which spread as far as Scotland before the order was dissolved by Henry VII.
Gilderoy Lockhart: Etym: The name of a famous robber, whose victims included Cardinal Richelieu and Oliver Cromwell.
Gilderoy Lockhart's Guide to Household Pests: Yet another of Gilderoy Lockhart's books.
gillywater (HBP ch. 15): A drink available to the students at Hogwarts.
gillyweed: A magical plant that allows someone who eats it to breathe water.
Etym: Invented; OED has an entry for gillyflower, meaning various pinks or wallflowers, particularly Dianthus Caryophyllus.
Ginger Newt (OotP ch. 12): A kind of cookie that Professor McGonagall keeps a supply of in her office.
Etym: A variant of the name we know best as Guenivere, originally from a Celtic root meaning "white, shining".
Gladrags Wizardwear: A chain store with branches in London, Paris, and Hogsmeade.
Gladys Gudgeon: Etym: From Welsh Gwladys, of unknown etymology. Folk etymology favors a formation from Claudia.
gnomes: In these books, burrowing pests that tear up wizard gardens. Plaster statues of gnomes are fulfill roughly the same function in British gardens as plastic flamingoes do in American ones.
Gobbledegook: The language of goblins.
Goblet of Fire: The magical artifact that selects champions for the Triwizard Tournament.
goblins: The bankers of the wizard world. As a species which specializes in handling money, they are required to be short, ugly, and unpleasant in accordance with the long tradition enshrined in the Secret Protocols of the Elders of Science Fiction and Fantasy.
Goblin Liaison Office: A department of the Ministry of Magic.
Gobstones: A wizard variation of marbles in which the marbles can spit a foul-smelling liquid in a player's face.
Godric Gryffindor: Etym: From Old English god, which may mean "good", + ric "ruler".
Godric's Hollow: Where Harry's parents were living when they were killed by Voldemort. A fictional place.
Golden Snitch: A tiny, winged ball used in Quidditch. It flies around the field of play attempting to not get caught by the Seekers. Catching the Snitch earns 150 points for the catching team and ends the game.
Goldstein, Anthony (OotP ch. 10): A Ravenclaw prefect in Harry's year and a member of Dumbledore's Army.
Etym: Ashkenazic Jewish for "gold stone".
Golgomath (OotP ch. 20): The new Gurg of the giants.
Etym: Might be related to Golgotha, Aramaic for "skull".
Golpalott's Third Law (HBP ch. 18): The antidote for a blended poison will be equal to more than the sum of the antidotes for each of the separate components.
Etym: Probably supposed to sound like "gulp a lot".
Gordon (PS ch. 3): A member of Dudley Dursley's gang.
Goshawk, Miranda (PS ch. 5): Author of the Standard Book of Spells series.
Etym: Astur palumbarius and relatives, primarily short-winged, forest-dwelling accipters. All the other textbook authors named have names clearly relating to the subject of their books, but I'm at a loss to explain this one. A pun on a real-world author's name, perhaps?
governors, board of: Some sort of oversight board for Hogwarts. Lucius Malfoy is (or was) a member.
Goyle, Gregory: The slightly stupider of Draco Malfoy's cronies, now a Beater on the Slytherin Quidditch team.
Etym: A deep trench or ravine. Or maybe intended to make us think of gargoyle.
Goyle senior: A Death Eater, still at large.
Graham Pritchard: Etym: From the surname of a prominent Scottish family, originally from the placename Grantham, etymology unknown.
Granger, Hermione: One of Harry's two best friends, and the smartest student in the school; a fighter for truth, justice, and getting those lazy slobs around her to do some real studying; even more of a force to be reckoned with now that she has the authority of being a prefect.
Etym: A word for a farm bailiff, responsible for overseeing the collection of rent. In the US, the Granger movement fought on behalf of farmers against the monopolistic grain transport practices of the railroads after the Civil War.
Grawp (OotP ch. 30): Hagrid's younger full-giant half-brother.
Etym: No idea.
great grey owl: A large owl distributed all around the northern hemisphere.
Great Hall: The biggest indoor space in the Hogwarts castle; where the students and staff usually dine. The ceiling is enchanted to look like the sky outdoors.
Great Hangleton: A town next to Little Hangleton.
Great Humberto, The (PS ch. 3): A stage magician (well, one assumes...) whose TV show on Monday nights is one of Dudley Dursley's favorites.
Great Wizarding Events of the Twentieth Century: A book Hermione read to catch up on wizard culture.
Great Wizards of the Twentieth Century: One of the books in the Hogwarts library.
Greengrass, Daphne (OotP ch. 31): Another student in Harry's year.
Etym: An actual recorded surname. Means what it looks like. I guess she's really good at Herbology.
greenhouse one: The only greenhouse first-year Herbology students are allowed into.
greenhouse three: A place with much more interesting plants than greenhouse one.
Gregorovitch (GoF ch. 18): The maker of Viktor Krum's wand.
Etym: Russian name, a patronymic (but not usually surname) from Gregory.
Gregory Goyle: Etym: See below for one meaning. In common usage, has meant "a gallant" (c. 1599), "a hangman" (17th century), and a children's game. My money's on the hangman...
Gregory the Smarmy (PS ch. 9): A statue that conceals a secret passage out of Hogwarts.
Etym: As a name, from a Greek word meaning "to be watchful".
Greyback, Fenrir (HBP ch. 2): An infamous werewolf and Death Eater, captured after the fight at Hogwarts.
Etym: I think it's supposed to be literal.
Grey Lady: The house ghost of Ravenclaw.
Grim: A death omen in the form of a huge black dog. Though it should also be noted that there is a "Church Grim", said to guard graveyards from witches and the Devil.
Grimmauld Place (OotP ch. 3): The street on which Sirius Black's ancestral home is hidden.
Etym: Well, the house is a grim old place, y'see...
Grindelwald (PS ch. 6): A dark wizard defeated by Dumbledore in 1945, likely on or before May 7th. (The mess in the Pacific was presumably still the work of mere Muggles.)
Etym: Grindel means "fierce, angry" and wald is German for "forest". Grindelwald is also the name of a place in south central Switzerland.
grindylow: A malignant creature that lurks in water, waiting to strangle the unwary. From Yorkshire folklore, where it is said to prefer deep pools of stagnant water for its resting place and children fr its victims.
Gringotts: The British wizards' bank, run by goblins and based in Diagon Alley.
Griphook (PS ch. 5): A goblin working at Gringotts.
Etym: Probably invented.
Griselda Marchbanks: Etym: Related to an archaic word for "gray, grizzled". Also, the name of a character in medieval romance who was celebrated for her patience and wifely obedience.
Growth Charm (OotP ch. 31): Mixed up with a Color-Change Charm by Harry during his O.W.L. test.
Grubbly-Plank, Wilhelmina (GoF ch. 24): The substitute Care of Magical Creatures teacher.
Etym: No info; probably one of those random interesting names off of a gravestone.
Grunnings: The drill-making firm of which Vernon Dursley is the director.
Grunnion, Alberic (PS ch. 6): A person featured on a Famous Witches and Wizards trading card. No further info found.
Gryffindor, Godric: Etym: Invented. The griffon (or gryphon) is a symbol of bravery and guardianship.
Gryffindor House: The house that Harry belongs to; it selects for bravery and leadership qualities. The house ghost is Nearly Headless Nick, and the head of the house is Professor McGonagall. Badge: Gules, a lion rampant to sinister Or.
Gubraithian fire (OotP ch. 20): Everlasting fire. Hagrid and Maxime presented a branch of it to Karkus.
Gudgeon, Davy (PoA ch. 10): A former Hogwarts student who nearly lost an eye to the Whomping Willow.
Gudgeon, Gladys (CoS ch. 7): A big fan of Lockhart's, still writing to him even during his rehabilitation.
Etym: Nickname deriving from a fish (Gobio gobio), which originally may have been used with reference to a greedy or credulous person. Also, various meanings referring to pins or sockets at pivot points.
Guidelines for the Treatment of Non-Wizard Part-Humans: A directive of the Ministry of Magic which covers vampires, among others. This of course brings up the question of the legal status of a part-human who is a wizard...
Guide to Advanced Transfiguration, A: Harry's fourth-year Transfiguration textbook.
Guide to Medieval Sorcery, A: A book in the Hogwarts library.
Gulping Plimpies (HBP ch. 20): Someting to be warded off with a Gurdyroot.
Etym: No idea.
Gurdyroot (HBP ch. 20): A root bearing a suspicious resemblance to a green onion. It supposedly wards of Gulping Plimpies.
Etym: No idea on this either.
Gurg (OotP ch. 20): The chieftain of the remaining giants.
Etym: Maybe as in gurgle?
Gwenog Jones: Etym: Couldn't find any.