Google Pay is available in version 3.0.5+ of the Magento 2 module.

In order to test the entire user flow on an Android device, the user must have at least one card stored in either Android Pay or their Google account. The user can also add a card to their account during the checkout process if none is present. In the sandbox, Google Pay will return valid testing nonces that point to a virtual account number; the card selected within Google Pay will not be charged. You can simulate server behaviors by using test amounts on your transactions and test nonces. Transactions will show in the Braintree gateway as Google Pay transactions immediately upon completion.

In production, you'll need to follow Google's documentation to register your domain and receive a merchant ID.

For more information please refer to Braintree's support documentation.