Unable to verify gmail on Mail?
I know this works on Sierra...
1. Go to your Google account settings (myaccount.google.com) and enter the security options.
2. Make sure the two-step verification is activated. Otherwise follow the steps to turn it on.
3. Right under the two step verification, enter the Apps Passwords option.
4. Choose "Mail" as app and "Mac" as device. Click Generate.
5. You will see a new password in the screen. Copy it and enter it in your Internet Account options in System Preferences in your Mac, and wherever it still asks for your Mail password. Make sure you keep this password somewhere safe.
MacBook Pro 13″, macOS 10.12