Note: Many of the entries here will eventually become full-length posts. Some are rough and have not been fully researched. If you have any corrections or would like to add anything, please comment.
Gage, gauge, gouge: Gage is rare. It means something given as security for an obligation. Gauge relates to measurements. Gouge relates to chiseling, scooping, and digging.
Galumph: a verb meaning to move clumsily.
Gambol: (to playfully skip or frolic about) makes gamboled and gamboling in American English; gambolled and gambolling in other varieties of English.
Garter snake: not garden snake, though there’s nothing illogical about calling a snake that lives in the garden a garden snake.
Gait vs. gate: Gait: a style of walking. Gate: primarily, a structure that opens and closes to permit entry and exit.
Gallop: makes galloped and galloping in all varieties of English.
Garrote vs. garrotte: (to strangle with cord or wire) garrote in the U.S.; garrotte everywhere else.
Gazebos: not gazeboes.
Gemology vs. gemmology: (the study of gems) gemology in the U.S. and Canada; gemmology outside North America.
Genera vs. genuses: The Latin plural of genus is genera. In English, genuses would be an acceptable form, as genus has been in the language long enough to have earned an English plural. But the English plural has yet to catch on, and genera is still preferred even outside science writing.
Generalise vs. generalize: In the U.S. and Canada, it’s generalize, generalized, generalization, generalization, etc. Outside North America, it’s generalise, generalised, generalising, generalisation, etc.
Genius: Don’t listen to anyone who says the plural is anything but geniuses.
Geographic vs. geographical: Both are common throughout the English-speaking world. They share all their definitions.
Geometric vs. geometrical: The longer form isn’t wrong, but the shorter one is preferred by a large margin throughout the English-speaking world.
Gestalt: a composition of elements that can only be appreciated as a whole rather than as a sum of its parts.
(From the) get-go: from the start.
Get the ball rolling: to start a process with the expectation that others will help keep the process moving forward. It usually takes the preposition on.
Get-together: (with a hyphen) a casual social gathering. When get together is a verb phrase (e.g., let’s get together), it is of course not hyphenated.
Ghetto: The plural is ghettos, not ghettoes. The word becomes potentially offensive when used as a pejorative adjective.
Girl: a female child. Don’t use it in reference to a grown woman.
Girlie, girlish, girly: Girlie is a diminutive of girl. It’s only a noun. Girlish is an adjective used to describe people. Girly is an adjective used for inanimate things.
Gist: not jist.
Go ahead and: has an astonishing array of meanings. Usually bears removal.
Good vs. well: Both work as adverbs in informal speech and writing. More formally, good is an adjective and well is an adverb. That’s the traditional distinction, anyway.
Goodbye, good-bye, good-by: Goodbye is the preferred form throughout the English-speaking world.
Gossip: Ignore the rumor that gossipped and gossipping are the preferred forms in British English. It is not true. Gossiped and gossiping are the preferred forms throughout the English-speaking world, and this has been so for centuries.
Grammarist: not a dictionary-approved word. A person who specializes in grammar is a grammarian.
Grasping at straws: trying things out of desperation or without a plan.
Grassroots: truly from the bottom up—without manipulation or aid from the top.
Grateful: greatful is a misspelling.
Gratify: Its original meaning is to satisfy or please, but the old sense has been poisoned by long-time use of the word to mean to please sexually.
Ground breaking, ground-breaking, groundbreaking: The two-word and hyphenated forms are sometimes used for the breaking of ground at the start of a construction project, but the one-word form has gained prevalence and is the most common spelling for all senses of the word.
Groupthink: the act of thinking as part of a group, especially at the expense of one’s individuality
Gruntle, gruntled: They exist only as latter-day backformations from disgruntle.