Maybe they'd prefer it for aesthetics, but OTOH in iOS 18.2+ they support off-device ChatGPT and apparently refer to it as "ChatGPT" both in settings and when prompting the user to ask if they want to use it.

If they do refer to it as "Gemini" then this is a huge win for Google, and huge loss for OpenAI, since it really seems that the "ChatGPT" brand is the only real "moat" that OpenAI have, although recently there has been about a 20% shift in traffic from ChatGPT to Gemini, so the moat already seems to be running dry.