Thanks for your update. I'm also testing on Outlook for Mac, so what's the version of your Outlook for Mac(), please first ensure that you have updated to the latest version.
Could you configure other pop account on your Outlook(such as Outlook.com or others)?
If you could add other pop account in your Outlook, the issue may be related to your Gmail account itself, have you turned on the 2-step verification of it? It's suggested to check your account setting in your web mail and created the App password to test if your account could be added normally with it in Outlook(Manage your Goggle account>Security>Sign to Goggle>2-step verification).