There could be a few reasons why you're having trouble logging in to Gmail via Mail on your iOS device. Some troubleshooting steps you can try are:
-First, make sure that you have the most up-to-date version of the Gmail app installed on your iOS device. You can update the app by going to the App Store and tapping on the 'Updates' tab.
-If you're still having trouble, try deleting and re-adding your Gmail account to Mail on your iOS device. To do this, go to Settings > Passwords & Accounts > Add Account, and then select 'Google' from the list of account types. Enter your Gmail login credentials and tap 'Sign In.'
-If you're still having issues, you can try troubleshooting your account in the Gmail app. To do this, open the app and tap on the hamburger menu icon in the top-left corner. Tap on 'Settings' and then 'Your account.' From here, you can select 'Manage your Google Account' and then 'Troubleshoot.'
Hopefully one of these troubleshooting steps will help you login to your Gmail account via Mail on your iOS device.