Gadflies:
An exclamation fo annoyance at flying insects. – Joseph Leff

Gag Reflex:
Laughter. – Joseph Leff

Gaggle:
A small joke. – Joseph Leff

Galahad:
The girl I used to date. – Joseph Leff

Galleon
Fuel measure for old ships

Gallery
A home for girls

Galling
A French fish. – Joseph Leff

Gallows Rope:
Noose reel. – Joseph Leff

Gallows
Where no noose is good noose

Gallstone:
To intoxicate a Frenchman. – Joseph Leff

Gamekeeper
Teacher who confiscates computer games in class

Ganglions:
A pride. – Joseph Leff

Gangrene
A group of Martians

Gardener:
A plant manager. – Joseph Leff

Gargoyle
Pulling faces when gargling

Garlic
Vegetable which is excellent for avoiding cold germs. Eat enough of it and people with colds will stay far away from you. Of course, so will other people.

Garter Snake
A perfectly harmless reptile, but it has been known to snap on occasion.

Gary Trudeau:
Gary, the money isn’t counterfeit. – Joseph Leff

Gaslight:
Petrol with only half the usual calories. – Joseph Leff

Gaucho:
A South American descendant of one of the Marx Brothers. – Joseph Leff

Genealogy:
(1) The study of denim trousers.  
(2) The study of Barbara Eden. – Joseph Leff

General Assembly:
A meeting of high-ranking officers. – Joseph Leff

Generally
An Arab general

Generation Gap
The distance you keep behind your parents when they do something embarrasing, like dancing in supermarkets