The bustling free city of Novigrad has a few extremely proficient Gwent players, each with their own unique cards that they are apt to wager. As you may have guessed, you can stride up and speak to them about other matters, and then choose a game of Gwent as it comes up in conversation. Or, you can find clues to a possible player by reading notices on boards throughout the city.

Look for the notice board in Hierarch Square, mentioning that “Vimme Vivaldi” is a cheat. Near by is the bank (where you can also exchange orens for crowns) and Vimme. Challenge and beat him in Gwent. A subsequent Novigrad player becomes available, thus starting the quest officially.