Just sideload the app on your watch and you can then long-press the bottom button on your Galaxy Watch 4/5/6 to launch Google Wallet (install it from the Play Store if not preloaded).
NOTE: Requires a watch reboot to take effect.
NOTE: Only works on watches shipping without Samsung Pay, i.e. only for watches sold in certain regions. For other users, use GW4Remap instead.
NOTE: On Wear OS 4.0 (One UI 5), the user needs to grant the app the SYSTEM_ALERT_WINDOW permission in order for it to be able
to launch the Google Pay app from the background.
adb shell pm grant com.samsung.android.samsungpay.gear android.permission.SYSTEM_ALERT_WINDOW