Are the cards that I have already added to my Samsung phone for Samsung Pay also available on the Gear smartwatch?

No.  You cannot directly import cards which have been enrolled in Samsung Pay.  Adding a card to Samsung Pay on your Samsung phone doesn’t automatically add the card to your Gear smartwatch.  Conversely, adding a card to your smartwatch doesn’t automatically add the card to your phone.  However, the same card can be added separately to both your Samsung phone and the Gear smartwatch.  Or, you can add a card to your smartwatch and not add it to your phone.