Beowulf? Nope. Beowulf and Grendel? Forget about it. This baby is called Grendel, and that's what it's all about: Gardner's title places the focus squarely on the monster. Why put that boasting, muscle-bound slave to honor in the spotlight again? That would be so 8th century.
The point of Gardner's narrative experiment is to see what happens when we take a character that is far beyond the pale of society and make him the center of the story's universe. At the same time, by riffing on the title of the Anglo-Saxon poem, Gardner gets to carry over all the thematic baggage he wants into his contemporary narrative—and then play around with it and subvert it. It's kind of a win-win situation.