Your gmail account is not set up correctly on your iPhone. Most likely you changed your gmail password or enabled 2 factor authentication on your gmail account, and didn’t update gmail settings on your iPhone to match.
If you access gmail using the Apple Mail app:
- Open Settings
- Tap on Apps (if you have iOS 18)
- Tap on Mail
- Tap on Accounts
- Tap on gmail and delete it
- Tap on Add Account
- Add gmail back so it can retrieve the appropriate credentials
If you use the gmail app:
- Open gmail
- tap on your icon in the upper right corner
- Tap on security settings
- Fix whatever it says is wrong