That would be as simple as using "thunderbird", setting it to gmail and using it as your default mail instance.
I think other IMAP capable mail clients like Sylpheed or even Mutt will do the same ...... just NOT the web version through your browser.
We could easily create a widget using webkit (similar to the stock-exchange widget) to run web-gmail but it would be a major security risk and a lot of unnecessary work/code to maintain.
Of course there's nothing stopping you from installing gnome-gmail and test if it will work. The app is in the repositories after all, so should work as a stand-alone mailer..