First, make sure that you've enabled IMAP or POP for your Gmail account. Next, select a storage option, whether you want your emails to be sent immediately to your iPhone or kept on your computer. Adding your Gmail account to iPhone should be relatively simple, but keep in mind that you may be blocked from accessing your email if Google detects suspicious activity. This will prevent you from receiving important messages.
See also - https://www.postpuff.com/the-complete-process-to-access-gmail-on-an-ios-device/
See also - https://www.postpuff.com/the-complete-process-to-access-gmail-on-an-ios-device/