You should be able to change any notifications for any app within the menu of the app.
The stock GMail app has gotten tricky lately. When you're in GMail, tap Menu --> Settings, then tap on the Account... then tap Manage Labels, once there, tap Inbox. It is only then that you'll see where you can set the notification ringtone.
This is only for the stock GMail, but the rest of the apps are similar and not as complicated to find the setting.
I hope this helps.