At least Barclays, Natwest and Halifax are not supported by Galaxy Pay but yes by Google Pay. On Samsung mobiles, Google Pay works ok but not on the phone... I can't understand it! Any unbranded android activity tracker has Google Pay working ok...very dissapointed
Just had the same problem at ASDA. I tried three different scanners, none would accept payment with my Gear S3. My Gear S3 worked fine in Sainsbury's and Morrisons the previous day.