That’s what inspiration is. No one has a unique thought, they subconsciously take ideas from several other concepts and piece them together.
There is no harm in prompting an ai model such as ChatGPT for game ideas. Plus you do realise that you can… be specific? You can feed it an initial thought or concept and ask it to expand on it, you can give it as much detail as you want but ask it to structure it in a proper way. It’s not cheating.
It’s not like you are generating the game using ai, no, you would be making the game, programming it, building it and so on.