The Witcher 3: Wild Hunt - Complete Edition

If you have a problem with unlocking this achievement, try reloading a save before completing this sidemission.
When I needed my last Gwent card I saved right before it. Because my achievement didn't unlock the first FOUR times... The day after I tried it once more, loaded up the save and then it popped!
I also collected the base deck first, didn't start the tournament yet and then start hunting all the single cards in the mission "Gwent: Never Fear, Skellige's Here" after triggering it from the quest entitled “The Beast of Toussaint.”

This achievement is related to the "Gwent: Never Fear, Skellige's Here" and "Gwent: To Everything - Turn, Turn, Tournament!" secondary quests in the Blood and Wine DLC. Unlike the similar quest from the base game, this one provides the actual location of the gwent players on the map. They are scattered across Beauclair, the Tourney Grounds, the Cockatrice Inn, Castel Ravello, Francollarts, and Flovive, but all locations are within a relatively close distance; just hop on Roach and start following the directions. Gathering all 19 cards is nowhere near as time-consuming as the achievement from the base game. The achievement should pop once you defeat the last player.

A decent Northern Realms deck should help you win all games without too much trouble. Just be careful with players using the new Skellige deck since the new mechanisms might take you by surprise.