Make sure you have the Google Pay app installed on your device.
You can select the Revolut card you wish to set up in Google Pay directly from the Revolut app. In the app, go to the 'Cards' tab to select your card. Find the card you wish to add and click the 'Add to G Pay' button.
You can also enter your Revolut card information in the Google Pay app. In this case, there may be a temporary authorization hold, which may take up to 15 days to refund.