custom gpts just edit the system message and the function instructions (called actions here), and adds some basic RAG functionality maybe.
the system message is the same as a user message and an assistant message, but the role is “system” - GPT-4 is supposed to listen to that more than to user input. these are just instructions.
a fine-tune actually changes the weights of the model.
maybe you can technically theoretically potentially achieve similar results with both methods, but the “Customized GPT” product is not using finetunes.
hope this helps