1. Open your Gmail app.
2. Swipe in from the left, scroll down, and tap Settings.
3. Tap your email account (between "General setting" and "Add account").
4. Tap Inbox Sound & Vibrate.
5. Tap Sound, then select your tone.
That will set the default tone for any email received on that Gmail account. But you can also set a tone for each inbox label. Under the Gmail settings, tap Manage Labels, then the inbox label you want, and then Sound to select the tone for that specific label.