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.
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.
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: The spirits of the dead which linger for some reason. 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.
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.
Ginny Weasley (Ginevra):
Etym: A variant of the name we know best as Guenivere, originally from a Celtic root meaning "white, shining".
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.
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.
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.
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 a member.
Goyle, Gregory: The slightly stupider of Draco Malfoy's cronies.
Etym: A deep trench or ravine. Or maybe intended to make us think of gargoyle.
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.
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.
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 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.
greenhouse one: The only greenhouse first-year Herbology students are allowed into.
greenhouse three: A place with much more interesting plants than greenhouse one.
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".
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.
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.
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.
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.
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.