As far as I know, I’ve not seen any FOSS alternative to Google Pay, and even propietary alternatives are somewhat limited.

WalletPasses is an alternative for holding membership passes or loyalty cards but doesn’t do credit cards.

Since it’s the business of money, you need to have money for big companies like VISA, banks, and money transfer services tall over the world to co-operate with you. There isn’t much incentive to provide their slightly more secretive APIs into a FOSS app.

Also, some might be worried about the easily modifiable nature of open source, it could be used to create a version of the app manipulated maliciously to steal your data.