ChatGPT is currently integrated into Apple Intelligence. When I ask Siri something I can choose to use ChatGPT for my answer.

https://support.apple.com/guide/iphone/use-chatgpt-with-appl...

So I'm guessing in a future update it will be Gemini instead. I hope it's going to be more of an option to choose between the 2.

Hopefully it gets more tightly integrated.