Gorgon - Three snake-haired sisters who can turn anyone to stone by making eye contact. Sometimes described with tusks and wings. These sisters are known as Medusa, Stheno, and Euryale
Griffin - A creature with the body of a lion, but the head and wings of an eagle
Golden Fleece - A golden fleece guarded by The Colchian Dragon, retrieved by Jason.
Gaia - Goddess personifying the Earth.
The Gray Sisters - Three sisters that share an eye and a tooth
Tell me if there's anything/anyone I missed!