After registering on Google Pay, you can open the app to find your UPI ID. 

Your UPI ID will have either @oksbi, @okhdfcbank, @okicici, or @okaxis at the end.    

1) Log in to the Google Pay app

2) Click on the 'Profile' icon on the right hand side

3) Click on the Payment methods 

4) Select the bank registered on Upstox if you have added two or more banks on Google Pay.

5) Check the UPI ID, under the manage UPI IDs