How is Grendel an anti-hero?
In short, Grendel, as he is portrayed in John Gardner's Grendel, is a point of view that subverts the status quo.
Grendel reveals that humans fool themselves by rewriting history, by applying meaning where meaning doesn't exist (in the signs of the zodiac, for instance), by creating gods where they don't exist. Humans constantly lie to themselves in order to emotionally and mentally survive and continue to live.
Grendel is the teacher of reality in the novel. He is the bringer of truth. He is the destroyer of illusion. He thinks about killing the queen in order to teach the humans reality. He refuses to kill Unferth and instead carries him back to the mead hall, in order to destroy the myth of the great, superhero-like hero.
Grendel points out the foibles and shortcomings of human society. He is a rebel and an outsider.
These make him an anti-hero.
At the same time, you should remember, he is also an unreliable narrator and he, himself, takes on the role of his nemesis, the Shaper. Grendel becomes an artist, a rewriter, if you will. He writes the novel. He tries his hand at poetry. He embraces art, even though he despises it. He both loves and hates art. And though Unferth is his goat, so to speak, he also reveals a kind of nobility--a kind of heroism--in Unferth. Humiliated, embarrassed, sinful (killed his brother), Unferth is still the warrior who sleeps in front of the queen's door to protect her.
Maybe humans do lie to themselves in order to survive. But they do survive and endure. The novel is about reality vs. art. And art wins.