Have you run into this Known Bug?
https://support.microsoft.com/en-us/office/unable-to-add-a-gmail-account-to-classic-outlook-4d9530c5-3f40-4e6d-9b45-4477f160828f
One workaround that seems to fix if for some people is the part about clearing your Windows Credentials. But work your way thru the whole thing.