Google Pay (combination of Google Wallet and Android Pay) was officially available on Wear OS (formerly Android Wear) only in a couple of markets so far: the US and the UK. That changes now, as the service has expanded to three more countries, including Spain, Australia, and Canada.
That'd explain why some users outside of the US and UK (see Reddit link below) are now able to access the functionality.
If you remember, Oreo for Huawei Watch 2 disabled the feature (yes, it was unofficially available earlier) in non US and UK markets. However, now users in at-least Spain, Australia, and Canada will again be able to use their watch to make payments through Google Pay.
There's currently no information on which other countries will get the support and when. For its part, Google just says more countries will be added to the list soon.