Should custom instructions be enough?

Custom instructions help one account. They do not keep a whole team, or a different model, on the same brand.

Does this apply to Claude and Gemini too?

Yes. Any model that starts from a blank prompt will drift. The profile has to travel with the request.

How do you keep ChatGPT on brand?

Keep ChatGPT on brand by giving it one saved identity: voice, colors, and offers. A clever prompt works once. The next chat forgets it. Review the output before you publish, and reject anything that does not match the profile.