I can't get Gmail set up again on my MacBook running High Sierra 10.13.6

Google has strengthened its security and older operating systems are no longer directly compatible.

To connect your Gmail account on your Mac:

1. First of all, delete the Gmail account.
2. Then, you will have to create an application password: Google - Application Password.
3. 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. If you do have it, try instead this direct link: https://myaccount.google.com/apppasswords.

Great! You are most welcome!