He rode with the Rohirrim to the relief of the city of Minas Tirith and was killed during the Battle of the Pelennor Fields, with the king's banner in his hand. His name is mentioned in the Song of the Mounds of Mundburg.
Gúthlaf was an Old English name, which Tolkien got from the Beowulf poem.
- The Lord of the Rings: The Return of the King, "The Battle of the Pelennor Fields" pgs. 838, 843 & 849