[silvertank]

If you click on Menu -> Settings -> Sound and select notifications, this is where you will set the default sound for any notifications other than phone ringer.

If you want Gmail to have it's own sound, open the Gmail app and go to it's settings and change it there. Same goes to all other apps you have. They each have their own notification settings in them.

I have 2 gmail accounts and i can set a different tone for each account by choosing the mailbox, then going to it's settings.