Just set up your gmail account in Mail and disable the .Mac account.
Goto Mail -> Preferences -> Accounts
To add gmail: Press the + button on the bottom of the window. Choose POP account. Enter your details (see attachment 1) under Account Information. Under Outgoing Mail Server drop down menu, choose Add Server and fill inn the details there, too (see Attacment 2). (Directions should also be available on gmail.google.com.)
To disable .Mac: Choose the account, goto the Advanced tab and uncheck the enable account.