I ran into this problem last night on my son's iPhone. He needed to sign into the Pokemon Go account we use that I made years ago under my email address. However, the Google sign in option wasn't appearing on the Pokemon Go sign in screen on the app.
If you're not seeing the Google sign in option for Pokemon Go for iPhone, the most likely reason is that when the app asked for the user's birth date on launch, a birth date putting the user under 18 was entered. There's no way to back out and enter a different birth date in the iOS version of the Pokemon Go app right now.
Follow these steps to make the Google sign in option appear in Pokemon Go for iPhone:
- Delete the Pokemon Go from your iPhone.
- Reinstall the Pokemon Go app from the App Store.
- Run Pokemon Go now, and enter a birth date that makes the user over 18.
- Now you should see a Google sign in option in the Pokemon Go app.