I’m not aware of any, but I hope to be proven wrong (edit: maybe PayPal - see below).

AFAIK, NFC payment apps have to be linked to your bank and have some kind of relationship with them. The app doesn’t just “spoof” your card over NFC (which would make card cloning terrifyingly easy).

So, any other Google/Apple Pay app would have to be setup similarly and have relationships with banks and card providers/payment processors. I don’t know of any third party services that have done that since there’s likely a lot of financial regulations around it (just guessing on that part).

Just realized PayPal might be able to fill the same role? https://www.paypal.com/us/money-hub/article/tap-to-pay

I haven’t tried that since there’s always a line of people, and I’m too embarrassed to cycle through several apps to test if they work at any particular retailer. lol

You may also check to see if your bank’s mobile app allows you to setup contactless payments.

But if you’re looking for a FOSS solution, I doubt there is one for the reasons mentioned above.