For some reason unknown to me, Opera does not come with the ability to select Gmail as the default mail provider. However you can fix this with a small configuration file change. After installing Opera open up C:\Program Files (x86)\Opera\defaults\webmailproviders.ini (if you are not running a 64 bit system the (x86) wont be present). Open the file and add the following text to the end:
Restart Opera after saving the file.
If you’ve already set up a different handler for MailTo links you’ll need to go to Tools > Preferences > Advanced > Programs. Then click the mailto protocol and click edit. Click the “Use Web mail service” radio button and Gmail should be present in the drop down list.
If you haven’t set up a mailto handler, just click on any mailto link and Opera will bring up a dialog box allowing you to choose Gmail.