@gianlucagiardina2
In first post you told that you had problems on Windows - now you say that on Windows all worked well.
On Ubuntu I have no experience at all, sorry. Do other Ubuntu browsers see and use your CA there?
Hello. Nice to meet you. It worked. Thank you. I was not sure if addon extensions from Google [Chrome] worked in Opera. I see they do! I see Checker Plus for Gmail and Gmail Checker Lite also work as well.
Logging into Gmail requires oAuth2 support now that Opera Mail doesn't support. However, someone said that if you goto https://myaccount.google.com/security, enable 2-step verification, create an app password for Opera Mail and use that app password to log into your Gmail account in Opera Mail, things will still work. You'll have to test.