You have to enable POP/IMAP on https://mail.google.com/mail/u/0/#settings/fwdandpop.
You have to turn on "Allow less secure apps" at https://myaccount.google.com/security#connectedapps. Or, if you have 2-step verification turned on at https://myaccount.google.com/security#signin, you'll have to set up an app password for Opera Mail at https://security.google.com/settings/security/apppasswords and have Opera Mail use that password instead.
Using IMAP with Gmail is preferred. When you add the account in Opera Mail, if your email address is not ...@gmail.com (as in, you have a Google Apps email), use ...@gmail.com in the account wizard so Opera picks the correct server settings for Gmail. You can then goto "Menu -> Mail and Chat Accounts -> select the account -> Edit -> General tab" and change it to what it's supposed to be. You still use your true gmail address for the username.
You can double-check your server settings if the account is already created. They should be:
Incoming Server(IMAP - Recommended): imap.gmail.com (or imap.googlemail.com)
Port: 993
TLS: Checked
Authentication: Auto
Username: your email address
Password: your password
Outgoing Server(SMTP): smtp.gmail.com (or smtp.googlemail.com)
Port: 465 (or 587, but 465 is preferred)
TLS: Checked
Authentication: Auto
Username: your email address
Password: your password
Incoming Server(POP): pop.gmail.com (or pop.googlemail.com)
Port: 995
TLS: Checked
Authentication: Auto
Username: your email address
Password: your password
Note: If you run your mail connections through your anti-virus, you might need to change the port and TLS settings for the servers. Otherwise, leave them alone.
To diagnose connection problems, you can turn on logging and check out the log file.