Overview
Gwent: Playing Thaler is a short secondary quest in The Witcher 3: Wild Hunt. It is added right after A Deadly Plot, but only if Geralt already played Vernon Roche before finishing that quest. Otherwise Thaler becomes part of Gwent: Old Pals instead.
What happens
You can challenge Thaler as soon as A Deadly Plot ends. If you leave, he moves to the Seven Cats Inn outside Novigrad. Beat him for the powerful Geralt of Rivia card. The quest fails if Thaler dies during Reason of State, so play him before that.