Adding a Gmail address to my Apple Pay account.

You will have to be more specific on what you mean by adding a gmail address to your Apple Pay Account.

• There is no Apple Pay Account.
• You can use a gmail address as your Apple Account email address.
• Cards added to your Wallet are used for Apple Pay, simply to process a transaction securely without the retailer knowing the actual account number of the card used.
• The cards in your Wallet are linked to your Apple Account and when you sign out of your Apple Account, all cards are removed from the device.
• If you are talking about Apple Cash, you can read more information here. You can add money to Apple Cash from a debit card or send/receive money to others who have an Apple Account who have also set up the Apple Cash card.

Add money to Apple Cash - Apple Support

Send and receive money with Apple Cash - Apple Support

There is nothing called an “ApplePay Card”

If you meant “AppleCash” …

There is no means to “add an address” to your AppleCash card.

It’s inextricably associated with your Apple Account.

If you meant “something else” please clarify.