Simply head for https://register.passport.net/reg.srf and register a passport using your Gmail mail address (you may have to sign out your current .net passport if it's automatically signed-in). Everything'll be done when you receive the confirmation mail and activate your account
Note that this has nothing to do with gmail logging in into msn. The two companies have nothing to do with eachother.
A .NET Passport can be ANY VALID email address...
If you have a valid email: myleetmail@my.crazyserver.co.uk you can make it into a .NET Passport also by registering at the site that Mnjul said using that email....