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.    

Log in to the Google Pay app

Click on the Profile button on the right hand side

Click on the Payment methods 

select the bank which is registred on Upstox if you have added two or more bank on Google pay

Select the bank which is registred on Upstox if you have added two or more banks on Google pay

Check the UPI ID, under the manage UPI IDs