Export you Gmail contacts via Outlook CSV format. Then, use http://hallvord.com/opera/conversion1.htm to convert to adr format.
If the downloaded file has a php extension, just change it to adr before trying to import into Opera (via Menu -> settings -> import and export).
Note: The website only supports convert a few main fields. Also see http://shadow2531.com/opera/wiki/How to Import Contacts from Other Mail Clients - Mail Client ( M2 ) - Opera Wiki.mhtml (download it and opening it locally for more info. Note that since mail.opera.com is gone, you can use that way anymore.
There are also some csv to adr converts that you can find searching google. But, I don't recall their names atm.