Gmailto No. 2 is a little app that allows you to set Gmail as your default email client in Mail.app, so that any email link you click opens a new email in Gmail’s web-based interface.
Download it, put it wherever you like and then open Apple Mail’s General Preferences. Select Gmailto (2) from the list like this and you’re all done:
As you can see from the screenshot, two Gmail notification utilities — GmailStatus and GmailNotifier — allow you to do the same thing, but if you don’t want to use either of them, this little app will get the job done.
It’s donation-ware and available from the developer’s web site 
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