When adding Google Pay, you will need to make sure that Google/Apple Pay app is already set up in your phone.
Go to Google Play Store and follow the prompts to download to your phone.
To add Google as a payment method:
1. Select "Wallet" in the menu in your Uber app
2. Tap "Add Payment Method"
3. Tap "Google Pay", then "Enable"
4. Follow the prompts in your app
To remove, just unlink Uber from your Google/Apple Pay app using your phone's App Settings.
NOTE: You cannot switch your payment method to Google/Apple Pay after a trip has ended.