Instances might occur when, in spite of entering correct login credentials,
you cannot log into your Gmail account on your iPhone or that the issue- Gmail not working on iPhone occurs.
You can resolve this by re-enabling the account access by going to the Gmail
website on your iPhone. Here’s what you need to do if Gmail not
working on iPhone.
Related Blog:
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How can I Speak to a live person at Google
Gmail Password Recovery without Phone Number
Read all above Official Blog Post To Fix Gmail Account.