In most instances, a return should work with Google Pay the same as it would if you had swiped a regular card. The key difference is that instead of the merchant having to swipe your card again you'll have to hold your phone near the payment terminal again to verify the return. Be sure to check that the card selected in Google Pay is the one that you used for the original purchase. In some cases you may also be asked for the last few digits of your card number. If this happens, you should provide the last few digits of your virtual account number. To view the last four or five digits of the virtual account number go to "card details" or "Transaction details.
Check with your retailer before making the return, as some merchants will require your physical card to process the refund.