I also have a Samsung Watch and have been able to get contactless payment working. Even though I don't use it, you should be able to use Google Wallet on the watch even when linked to a GrapheneOS phone, you just have to register the card on the watch only. However, I have to use Samsung Pay because my bank is run by idiots, and to get it working you gotta:
- Install the Samsung Pay add-on, preferably through the Play Store.
- Install an activity launcher.
- Use it to launch the main app activity for the Samsung Pay add-on.
- Now, it's gonna launch and error on you, telling you to contact customer support or smth. Just click the link that'll open the Samsung customer support webpage your browser, and wait up for like 10 or 20 seconds; just enough for the activity of the Samsung Pay app to get suspended and forget the check was unsuccessful. Ensure you're using an external browser, not an in-app one if this is not happening.
- Now you can use the back button to go back to the app, and you'll be able to navigate it for like a minute before it errors on you again.
- Speedrun adding the card, you're gonna need multiple tries, and if you're lucky you'll be able to repeat the trick without losing progress.
- Profit.
Now, this worked much better a year ago, now you gotta wait longer for the error message to go away while in the browser, and in general it's less reliable. However, I've been able to use Samsung Pay this way. Idk why you'd do this instead of using Google Wallet anyway, I've only had to discovered and been doing this because my bank is run by such idiots Google won't let them use Google Wallet unless they smarten up and do actual authentication.