This post has been sitting in the back burner for a while now, but I think I can just post it in it's current form.
In my campaigns, gargoyles are not evil creatures. They are in fact a form of benign fey with ties to the good gods and celestials, though they're forms are similar to demons and devils. They inhabit places also inhabited by men, protecting them from evil. They may have once been demons that were converted to good somehow, but this theory is unverified.
However, there are creatures similar to the traditional D&D gargoyles. They are constructs, created by demons in mockery of real gargoyles. Perhaps Demogorgon, infamous for his for his use of various constructs such as the Retriever commissioned the first of these, but that is not known for sure.