Gmail isn't a mail client, it is a mail service. You would need to install a mail client like Mozilla, and set it up to use your Gmail account, then set Mozilla as the default client.
I'm not experienced in this, but I'm going to guess that it can't be done. Setting a default Email client involves declaring an application to be opened for mail. Gmail is done through a browser, not a mail application.
I would not be surprised if Gmail provided either an app or a plugin to get around this, however. I'll peek at their support site.
Gmail isn't a mail client, it is a mail service. You would need to install a mail client like Mozilla, and set it up to use your Gmail account, then set Mozilla as the default client.
If you use firefox as your browser, go to options -> applications. Scroll down to "mailto" and change the dropdown box to read "gmail". Now if you click on a mailto address, it will take you to gmail.
That's about as good as it gets unless you use an email client like others have stated.