Gwent: Playing Thaler
Side quest
Novigrad 
On the world map
From the journal
Bernard Dukat, a.k.a. Thaler, was a truly great gwent player. What makes a gwent player great, you ask? Equal parts cunning and good cards, I reply. These last Geralt wanted for himself, for he felt his own collection was somewhat lacking.
As you might have guessed, Geralt managed to acquire Thaler's best card.