Etym: Probably from the meaning "a great prize or catch", referring to the capture of Spanish galleons by English privateers.
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, Peeves, and Professor Binns.
Etym: A variant of the name we know best as Guenivere, originally from a Celtic root meaning "white, shining".
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.
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?
Goyle: 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 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.
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.
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.
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 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.