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.
It seems to me that somebody tried to imply that a Geat was a Thane. This is not so, to clarify. Beowulf was a Geat (or Ostrogoth, as it were), who had a natural rivalry with the Danes. However, Beowulf was in the mind to pay the king back for a deed he did long ago (as well as to further his fame). Beowulf's Thanedom is a matter of his place in his own culture. A Thane is a man who pledges allegiance to his king and in that would share the spoils of war with the king. The reputation of a king always depended on the loyalty of warriors and Thanes and a reputation preceeding them, so a king would attract attention to himself by sharing the spoils of war, so a Thane was much like the feudal lords who ruled serfs later on in the medieval ages, or warlords. Beowulf followed the Germanic Heroic Ethos, or the code of the warrior, and so he disregarded this natural rivalry to help out his fellow man.
Beowulf is a Gate , he fight Grendel becuase he got a help request from Hrothgar , king of Danes.
Beowulf was a Geat who traveled to aid the Danes.