Google Pay is not supported by the Watch3. Google Pay is an Android based app. The OS on the Watch3 is Tizen derived, thus not compatible.
The Watch3 supports Samsung Pay but that with a limited number of card providers.
However, in the UK, the recently announced Samsung Pay Card powered by Curve is your answer. This makes Samsung Pay compatible with almost all banks and card providers.
I can vouch for this! Hope this helps!
🙂
Current Samsung Devices: Galaxy S20+ 5G (Cosmic Black/128GB) | Galaxy Watch3 (Mystic Black/45mm Bluetooth) | Galaxy Buds (Aura Glow Silver)