If you have a UK purchased device, then you should contact Samsung customer support and give them the details of the devices and ask what needs to be done for you to use Samsung Pay.
I believe it also depends where the seller purchased the devices, but i believe this can be checked for you by customer service.
Then this seems to be the reason most likely - your device is Non-UK. I believe you can call Samsung customer support and give them the details of the device and ask what can be done (if anything) for you to use Samsung Pay.
According to the Gear app, I have the latest version and it checked for an update yesterday. I can't find the version number. I get the same response as others have posted. If I click on Samsung Pay in the Gear settings, Installing Samsung Pay appears breifly at the bottom, the download icon appears at the top, the screen blinks, and I get back to the app without Pay.
I'll look into the manual install solution, but if they are aware of it, it should be fixed by now.
that's great! you can now use Samsung Pay on your Gear/Galaxy Watch?