Had this been a tech question like what happens if I assign value to a de referenced pointer I could have told its undefined behavior. Lets tag the pros, @Abhishek012 , @NShah
I think: If there is some other bank A/c, other than primary/default one, linked to G-Pay, it will get credited into that.
Otherwise, remitter will be shown pop-up that he can't transfer, it does happens in case of Paytm (UPI handle chane from @Paytm to partnered banks like, @ptaxis, @ptsbi) not so sure in case of G-Pay ot the txn will get failed or if money gets deducted somehow, then will get reversed.