I don't think you can restrict it, in fact, I think it's in your interest not to. GPT's ability to work with natural language comes from the large corpus it's trained on.

Adding specific product documentation would give it "domain expertise" of sorts (assuming this is what OP did), but training it ONLY on your documentation -- I don't know enough to call it, but my intuition is that it would weaken it's ability to interpret the users' questions.