I don't see the rape (or, for that matter, various other actions) as directly connected to his search for revenge, but rather the result of Gully Foyle being, essentially, a human animal (in the pejorative sense of that term); almost sub-human. The thing is, Gully becomes human; he evolves (almost in spite of himself) until what he was in the beginning ... and for some time thereafter ... is almost another creature entirely. He goes from a completely selfish, sociopathic monster to someone who develops a strong empathy for and faith in his fellow human beings. In some ways, he is thus related to Ben Reich of The Demolished Man, in that Reich, too, is a completely self-focused (albeit more polished and educated) individual; so much so that his own mind is blindered into misreading something, leading him to murder... a particularly vile murder, given the circumstances. In the end, like Gully, he is "demolished", deconstructed, disassembled as a personality, allowing him to begin the long climb into what he was potentially all along.
Gully is, really, both a very real, human character (complete with the sort of horrendous flaws which even very good human beings have been known to exhibit from time to time... and yes, that includes rape and murder), but also a symbolic character exemplifying the slow, gradual, and painful evolution from the animalistic to, in the end, something approaching godhood. I don't think it is any coincidence that Gully, like so many such figures, at the end of the story goes into a long sleep... to awaken at some future time as... what? It is in this growth from such a stunted creature to something holding much of what is noblest in humanity that Gully, as a character, "redeems" himself, in my view.