This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Gurangatch (Australian Aboriginal)
In Aboriginal Australian mythology, the gurangatch is a lizard/fish monster of incredible size that could swim through water and bore through stone. His travels created many of the continent's rivers and lakes. Gurangatch, not wholly a serpent, but part fish, and part reptile, camped in the shallows of an area known as Murraural, specifically at the junction of the Wollondilly and Wingeecaribbee rivers.
A fish-hunter, Mirragan the quoll, glimpsed the light reflected from Gurangatch's eyes and endeavoured, unsuccessfully, to spear him. The quoll tried to force his prey back from the depths of the waterhole, where Gurangatch had sought refuge, by planting ever more bundles of nauseating slabs of millewa hickory bark here and there in the various soaks and pools.