I feel I have to jump in here to defend the GAP series, like all Donaldson's books, it's about psychology. Ofcourse, some things that happen are really awful, it's not a books for little kids, but it's the transformation of the characters that makes it unique, not the fact that D is unusually graphic and cruel to his characters.
If you think about it, the three main characters start out as victim, bad guy and rescuer. They then change places as the story progresses, the bad guy becomes the victim and then the rescuer and the rescuer becomes the bad guy and then the victim. You get the picture.
When it comes to fantasy, the Covenant Chronicles are unusual. The main character is an anti-hero and not very likeable, that's true. If you want heroic knights, good versus evil and happy endings, you've got the wrong author. These books are different. And because you can't trust the author - he might kill off Covenant and have the bad guy win, you never know what's going to happen, which is what makes them great.
I would recommend both these series to anyone who have read a lot already and are getting bored with the same old stories. They are challenging, horrible, jawdropping books at times, but good just the same.
By the way he's also written the series Mordant's need, which is not bad. Starts with a woman sitting watching herself in the mirror because she thinks she'll disappear. Then ofcourse she gets thrown into a fantasy world and all that.