Exactly. @michaelchase gets it.
GPTs are a more customizable, and persistent wrapper for custom instructions. Sure, they are more capable than that with the increased functionality of adding knowledge documents and actions, but this is the framework from which we should be viewing their usefulness (at least for now).
All of the prompt based security methods mentioned above can be cracked. The best that anyone has been able to do so far is create a gamified version of security protocols that lock you out of doing anything with a GPT without specific steps being followed. What the practical utility is of that…I really could not tell you.
Any “secrets” you have need to be stored utilizing more advanced security protocols (data encryption etc). Use the Playground, get familiar with developing with API and with building Assistants outside of the scope of the ChatGPT interface. This is where you will be able to protect something you view as a truly unique product offering.