I have the Samsung Gear Sport which isn't supported, this is why I installed the APK. Hopefully someone else can assist and this thread can assist others in general too.
I have a google pixel 3 XL with a Samsung Gear Sport. Samsung Pay did not work and this was confirmed when calling customer services. To work with a google pixel 3 XL the Samsung pay plug in needs to be installed as per the above. Samsung Phone users shouldn’t have any problems using the offical option of set up.
Samsung Pay works/should work on Gear Sport. I used some time ago Samsung Pay on Gear Sport without any problem. The only thing that comes to mind is that you might have a non-uk device.
It doesn't actually matter what device you use (android/IOS) as Galaxy Wearable app is a normal app that works on Android/Ios and Samsung Pay (plug-in) works sits on the watch. I'll take a closer look to see how this works.