This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Gagana (Russian)
Gagana is a miraculous bird with an iron beak and copper claws featured in Russian folklore. She is said to live on the Buyan Island. The bird is often mentioned in incantations. It is also said this bird guards the Alatyr, alongside Garafena the snake. Gagana knows how to conjure and work miracles and, if she is asked correctly, can help a person. This bird is also the only one capable of giving milk.
The bird Gagana is possibly from a tale by A. A. Erlenwein, where the hero's sisters marry a bear, an iron-nosed bird ("uccello dal naso di ferro") and a pike ("luccio"). The "bird with iron beak" appears to be a creature that inhabits several Slavic folktales. Other descriptions are that on the mythical island of Buyan there lives "The Tempest Bird", "the oldest and largest of all the birds", said to possess "an iron beak" and "copper claws".