Google has strengthened its security and older operating systems are no longer directly compatible.
To connect your Gmail account on your Mac:
- First of all, delete the Gmail account.
- Then, you will have to create an application password: Google - Application Password.
- Finally, when you add the Gmail account in Mail on your iPad, choose "Other" and not "Google". Fill in your email address and application password.
You need to have 2-factor authentication activated on your Google account to be able to generate app passwords. If you do and have trouble accessing the link above, try this direct link instead: https://myaccount.google.com/apppasswords.