In total, there are 43 Gwent cards in the brand new Skellige deck. Like the base game, Gwent cards can be won by defeating Merchants, Traders, and Innkeepers in a match of Gwent throughout the duchy of Toussaint.
NOTE: None of the Gwent cards are missable.
The very first opportunity you'll have to grab a Skellige card is during the quest entitled “The Beast of Toussaint.” After a lengthy cutscene inside the Cockatrice Inn, take a few minutes to play Gwent against the Innkeeper.
After winning his randomly generated card, it will trigger a quest entitled "Gwent: Never Fear, Skellige's Here." This quest will help you track 18 more cards because it places persistent objective markers over every possible Merchant/Innkeeper you can win a card from across Toussaint.
In addition to this, the quest’s secondary objective will bring you to Beauclair, leading you to Count Monnier, the creator of the Skellige faction deck. Speak with him about his Gwent Tournament and refuse his offer to enter the tourney for now. He will hand over a base Skellige deck with 24 cards (23 standard cards and 1 Leader). You can return to him at anytime in order to enter his tourney, though take note only Skellige decks are allowed so it’s best to hold off on entering until your deck is near complete.