Yes it is available!!
Be very careful with the install though - the Samsung pay app on the phone is *not* the same as the app on the phone that runs for the gear(!!) I was using Sammy Pay successfully for couple weeks with no probems on Note4 paired with the Gear 3. Problems came when I moved to S8 plus - I made the mistake of thinking there is only one Sammy Pay app for the phone. So I installed Samsung Pay on phone then set up my credit card and 8 loyalty cards (yes it does those as well). That worked fine on s8 - made number of payments just fine (other than the annoyance that swiping up now brings up Sammsung pay - unfortunately its also the same action to bring up the home keys - number of times Ive started Samsung Pay app by mistake!
I then paired the Gear 3 to S8 (which performs a soft reset on the watch), which then asked me to install Samsung Pay on the phone (what? - I already had Samsung Pay set up), reset assured it prompts you to install the Samsung Pay for Gear Plugin - however the plugin is an identical app but just called Samsung Pay for Gear - and anything you already set up will not be in the Gear app on your phone.
I think mines got confused in the end I had to uninstall everything and reinstall - just dont bother adding any cards to the Samsung app until you have the Samsung App for Gear all set up first - also to make sure you are in the correct Samsung Pay app open the app on your phone from the Gear first, that way you'll save wasting time not adding your cards to the wrong app.
On the plus side, when it works it works just great! Especially when you see the confused look on the cashiers face.
Just make sure your bank support it tho. Its stated to work for cards from Nationwide, Santander and MBNA, HSBC, First Direct and M&S Bank. It worked for my Nationwide cards.
BTW, you can use Sammy Pay anywhere where they offer contactless payments. Also to note Samsung Pay has no limit, its up to the merchant wether they haver a limit on Samsung Pay - most will stick to the £30 limit to keep it in line with normal contactless card payments.
