Although Samsung Pay had been available on the market for over a year since the days of the Galaxy S6 and way before the Gear S2, for some reason it has yet to make its way to the Gear S2 until now.
Samsung is finally bringing the Samsung Pay to the Gear S2 via the recent Value Pack software update. Anyone with a Samsung Gear S2 smartwatch can now use their device to make payments on NFC contactless terminals. Using the Samsung Pay on the Gear S2 is pretty much straight forward, and requires long pressing the back key to launch the app and the holding it near the NFC reader to pay. Using it does come with a small caveat as one would have to re-enroll one's cards on the Gear S2 as the Samsung Pay app on their mobiles doesn't sync their card details over to the smartwatch. And also the Samsung Pay is currently limited to the Gear S2, users with the cellular version of the Gear S2 or the Gear S2 Classic would have to wait a while longer.
The Samsung Pay app on the Gear S2 only supports NFC payment systems and not traditional payment terminals as it lacks the necessary hardware for MST.