Many theorys has been said about the gimp, but it's all silent around Maynard and Zed. This is my theory about them all:
Zed's gay (we all know that) and Maynard is a sadist(not gay, becouse then he would have keept Butch and Marsellus for him self) who enjoys watching people suffer. The gimp is Maynards inbreed, retarded brother. We must remeber that he is a hillbilly.