Since they authorized the transaction, Regulation E error resolution procedures will not apply. You would have to look at any other protections offered by your card issuer (i.e. VISA, MC, Etc.).
V/MC do not provide any protections for person to person transfers. The customer received the requested goods and services from the merchant (Google Pay). The merchant sent the money in accordance with the customer's instructions. Sending money through online person to person transfers is the same as handing a stranger on the street an envelope full of cash and then being disappointed that the stranger never returned with what they promised you were buying from them.
Note that if a customer's debit card information is stolen (via phishing emails, phone scam, physical card stolen, etc.) and the fraudsters use the stolen information to establish a fraudulent Google Pay account to initiate transfers, Reg E would consider these to be an error as an unauthorized electronic funds transfer.