One frustrating thing about using Gmail for mail is clicking e-mail addresses won’t take you into Gmail. Fortunately, there’s two different ways you can accomplish this.
Google Notifier
The first and easiest method is using Google Notifier for OS X, which adds a Gmail icon in your menubar:

The Google Notifier is nice because it notifies you of new mail in addition to using GMail for e-mail links.

The only downside to this method is it requires Google Notifier to be running for the e-mail link to work. It will launch Google Notifier if not already running, which adds a small delay.
GmailTo
The other method is to use Gmailto, an application designed specifically for forwarding mailto: links to Gmail. I prefer this method because its small and efficient and doesn’t require any extra software launching.
After downloading Gmailto, open Mail.app Preferences (or Default Apps) and change the “Default email reader” to Gmailto.

Close and you’re ready to use Gmail as your default email client. Test using this link.
Matt
http://mail.google.com/support/bin/answer.py?answer=13275&topic=12810 March 25th, 2008 at 3:11 am
Brad Jasper
IMAP support is pretty good, but I've found using the Gmail interface is actually faster for me. March 25th, 2008 at 7:20 am
lee
Dave
Jon
Brad Jasper
I'm having the same problem recently. I could have sworn it opened a new compose window when I initially tried this. Will find out and report back.
- Brad March 28th, 2008 at 7:04 pm
Eliot
This is what I did, and it worked better than gmailto, since that apparently doesn't work well on its own in os x 10.4.8 or newer. August 14th, 2008 at 9:14 am