The Greyhawk wiki has a smidgen of info about Gadheylyn
Gadhelyn is an elven Hero-deity who was sponsored for ascension by an unknown patron. He is an elf of sharp features and long yellow
hair, and his eyes are bright, piercing green.
Gadhelyn teaches his few followers to seek joy and pleasure in all
seasons, and eschews social caste and status in favour of individual
merit and accomplishment. He is a hero still venerated among the
grugach, but otherwise mostly forgotten.
1d4chan has a smidge.
Gadhelyn (pronounced gad-THEL-len), is the elven and Oerthian Hero-God
of feasting, hunting, independence, and outlawry. Once a part of the
larger elven pantheon, the Lord of the Wildwood has mostly been
forgotten, with only the grugach worshipping him.
Rejoice in the springtime, make merry in the summer, feast in autumn,
and dream in winter's shadow. At the court of the Lord of the
Wildwood, knights are knaves and the rude are royal. Let all who would
enter the forest bring a gift to the Lord of the Wildwood; but beware,
if it is not pleasing, he shall take what he will in its stead. When
the first arrow strikes the heart, it is a noble wound; when a second
must be shot, it is fitting that the hunter should become the hunted.
Wikipedia has a smidge in its listing of Greyhawk deities.
Gadhelyn the Archer (Gad-THEL-en) is the elven hero-god of
Independence, Outlawry, Feasting, and Hunting. His symbol is a
leaf-shaped arrowhead. Gadhelyn is a very old figure in elven myth,
once a part of the Fey Mysteries but now largely forgotten except
among the grugach. He is depicted as an elf with sharp features, long
yellow hair, and vivid green eyes. He wears rough clothing of fur and
hide, of colors to match the season. Gadhelyn is still a potent hero
among the grugach. Sylvan elves and even a few half-elves and humans
revere him and participate in his rites. Followers of Gadhelyn prey on
the wealthy who dare to cross their woodlands, but they are not truly
dangerous unless attacked, or if their forests are despoiled.