The problem: security
This is one of the biggest concerns for many companies and consumers alike nowadays. Companies do not want to get sued, and consumers do not want to have their personal information stolen. When security becomes too tight and no explanations are provided, many find themselves locked out of their own accounts. The issue is not with the security, it is the lack of explanations that leave iPhone users fully in the dark.
How to fix Gmail on your iPhone or iPad?
Visit the Gmail website and check for alerts- We want to visit the Gmail website to get a better idea of what’s going on with Setup Gmail on iPhone because the mail app cannot give you any details about why you are unable to sign in.
Review your recent devices on Google's website- If you didn’t get an email about a blocked sign-in attempt, you can visit the section called Device activity and notifications on Google's my account website.
Do the CAPTCHA Reset- Gmail has a little-known fix called a CAPTCHA to reset which unlocks some of Google's security features to allow new connections to Gmail.
Read also - How do I set up IMAP for Gmail on my iPhone?