If you have syncing enabled killing gmail will only get rid of it temporarily. The next time it syncs, the gmail process will be started again. Best thing to do is to either:
Bring down the notification window, click Quick Settings and ensure Mobile Networks is switched off.
From your home screen tap the menu button. Go in to settings -> Accounts & Sync and disable the account you don't want to sync, or switch off the background and auto syncing.
Hope that helps.