gaga
Senile, with overtones of being crazy and/or foolish.

• "Completely gaga"
  - Rita Skeeter on Elphias Doge (DH2)

galoshes
U.S.: rainboots.

• The man with the watch wore a tweed suit with thigh-length galoshes; his colleague, a kilt and a poncho.
  - why it's unfortunate that Muggle Studies isn't a required subject (GF7)

gaol
U.S.: jail.

• the terrifying wizard gaol (GF2)

galumph
To gallop or prance. The word was created by Lewis Carroll in the book Through the Looking Glass.

• Harry's uncle came galumphing out of the living room (OP2)

gâteau
A large rich layer cake, where some of the layers are made of cream or fruit.

"Gâteau" is a loan word from the French gâteau, "cake", so strictly speaking the first 'a' should be written as â, but in common English use that detail is often omitted.

• "Chocolate gateau!"
  - Ron, trying to tempt Hermione into eating (GF12)

geroff
Colloquial form of "Get off", which in turn means "leave me alone" or "let go."

• "Mum - geroff!" (PS6)

• "Gerroff me! Gerroff me!" squealed the gnome (CS3)

• "Ouch - gerroff - gerroff, you mad old bat!"
  - Mundungus to Mrs. Figg (OP2)

gillyflower
Any of several plants related to the carnation that have fragrant flowers. This term crops up occasionally in descriptions of Elizabethan gardens. (AHD)

Compare with gillyweed.

git
Idiot. A variant of the slang noun "get," which also means an idiot (usually in Ireland or the north of England) and which was derived from a rather archaic term for the offspring of any animal (that, in turn, was derived from the verb to beget, meaning "to give birth to".

• "that old git"
  - referring to Filch (PS8)

• "an ignorant old git who lived like a pig" (DH22)

• "Little git," whispered Ron. "He's enjoying keeping us hanging (DH24)."

glen
A Scottish term for "valley".

• "Fair Ravenclaw, from glen"
  - Sorting Hat (GF12)

gorn
Gone.

• "We haven't got him, and they've gorn and killed her!" (DH30)

• "She's gorn and sent for him" (DH30)