In https://forums.opera.com/topic/28446/cannot-get-opera-mail-to-work, the pic shows that you created a POP account, but changed the POP server name to the GMAIL imap server. This won't work.
Delete the account in Opera and try again. If you use your gmail address in the new account wizard, Opera will automatically fill in the right settings. It'll do that whether you choose POP or IMAP. IMAP is usually what you want, especially in the case of Gmail.
But, make sure that you go to https://mail.google.com/mail/u/0/#settings/fwdandpop and enable IMAP. You'll also have to go to https://myaccount.google.com/security#signin and enable less-secure apps. Or, you can enable 2-step verification and set up an app password for Opera Mail and use that for the incoming and outgoing server passwords in Opera Mail.
