Wait a minute . . . isn't that a Hamlet reference?

Good works of literature often "borrow" images, references, symbols, and characters from other great literature and cultural myths in order to advance a certain idea and get us -- the reader -- thinking in new ways.  These allusions and reference-points remind us of larger ideas and themes and thus get us to experience the text in a different light.
What connections to other literature, myths, stories, cinema, and art do you see in Grendel?  Why does Gardner draw this parallel (what is the significance of this connection)?