I confirm that GarminPay works with the watch smoothly.
Additionally, what I liked about it is that you can make payments well over the maximum payment limit for contactless credit/debit cards. This is possible, I think, due to a PIN that you setup inside Garmin Connect app. When you wear the watch and decide to use GarminPay, you have to enter the PIN first time. You don't have to enter the PIN for consecutive payments as long as you don't take them off. It appears, that the built-in Heart Rate monitor is used to track when you take the watch off.