Well, first of all, Gmail is an ONLINE e-mail service and not a program, so technically, one cannot "set" gmail as a default e-mailing program.
There are two ways to access Gmail:
1) via browser (Internet Explorer, Firefox, Opera, Chrome, etc): http://mail.google.com
2) via e-mail program (Outlook, Thunderbird, the Bat, Eudora, etc..)
If you decide to use a program to access the online e-mail service, then once you have the e-mail program configured, you may set it as the default mail program through Windows 7 Control Panel (click start, type "control panel", hit "enter", click "Programs", then click "Default Programs" - the icon, the click "set program access and computer defaults" ---- here select the installed e-mail program you wish to use as your default).
I personally use Thunderbird to access my Gmail account. It's a great and versatile program.
If that's what you decide to do as well, here's a quick tutorial how to set up Thunderbird to access your Gmail account:
---> Thunderbird 2.0 - Gmail Help