Well, if you go into the Mail app, press Menu, press More, press Settings, you can go into Notification settings and turn off Email notifications.
That said, it sounds redundant to have your gmail account set up in both the Gmail app and the Mail app. If you do not need that, you will have noticed in the Settings that there is an option "Delete account". This will definitely stop the notifications.
(One reason you might want the Mail app to also check your gmail account - the People app will associate mail messages received in the Mail app with your contacts in the People app, but not with the Gmail app, so, if you use that feature, you'll want to keep the mail account set up, but turn the notifications off).