Gwent: Velen Players
Side quest
Velen 
On the world map
From the journal
Though you might think it unlikely, even war-ravaged Velen boasted many prominent gwent players. They possessed certain valuable cards which Geralt wished to add to his collection. To do that, he needed to defeat them. The first on his "To Fleece" list: the Bloody Baron.
In the end, Geralt crossed every name off his list and acquired all the valuable cards he sought.