Was Beowulf a Dane, Geat, or Saxon?
Beowulf is a Geat, also known as a Thane. The Geats are from what is now modern day Finland/Sweeden.
He fights Grendel for the Danes.
Beowulf is the leader of the Geats.
I assume you haven't read the poem. The poem states that Beowulf is a Geat. Do read the poem. I'm sure you'll find it worthwhile.