I Have a Gwent Problem is an achievement in Blood and Wine expansion and requires one to collect all the cards in the Skellige Gwent deck.

The easiest way to find them is to start the secondary quest Gwent: Never Fear, Skellige's Here, either by playing and winning a card from a player in Toussaint or by talking to Martin Monnier.

Tips

• While it says "all" - it means 19 particular cards and the base deck. Thankfully, once you pick up one of these (or talk to Monnier) they'll automatically be marked on the map.
• For a list of all players (including ones who don't count towards this achievement) see Gwent players.