This quest will only appear in your journal if the "Gwent: Old Pals" quest has been completed before the "A Deadly Plot" quest. Otherwise, playing with Thaler will be part of the "Gwent: Old Pals" quest. You'll get it after you escort Thaler at the end of "A Deadly Plot" to his wagon.
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.
