I prefer GMail web interface than any other desktop client. Out of the box you can’t select GMail web interface as default mail client in GNOME on Ubuntu. But you can get it done with gnome-gmail application.
Step-1: Download the latest installation package .deb from here and install it by double clicking it or through command line as shown below. As of now the latest file is gnome-gmail_1.5.1-1_all.deb.
$ sudo dpkg -i gnome-gmail_1.5.1-1_all.deb
Step-2: Select GMail as default application in System -> Preferences -> Preferred Applications. You are done!
Note: There is a plain hack available here. It may also be useful to you if you are a advanced user.