Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
First Person (Central & Peripheral Narrator): Grendel
Gardner likes to play around with the conventions of narrative, and we can see that pretty clearly in his choice to show us the other side of the Beowulf story. It goes further than that, though. Sure, Grendel gives us his first-person account of what it's like to be monstrous. But he also has this ability to know what other people are thinking—and he seems to see things that he shouldn't be able to see.
Gardner never moves away from Grendel's first-person experience, but he gives his creature insight into others' actions and motives. Think about those moments when he turns what he observes into a screenplay, or when he reads the intimate thoughts of Unferth. Maybe it's just great monster perception, or maybe Gardner wants to comment on the nature of existence.
Yeah, we totally just went there.
Okay, we won't weigh you down with too much philosophy, but it's worth noting that while Grendel doesn't have the kind of infinite vision the dragon has, he can still read between the lines enough to tell it like it is.