If you want to get all of the cards and do it efficiently, keep note of who you've already played. I do it on my "notes" app on my iPhone for games - actively noting who you've already beaten for their Gwent card will save you from running around trying to figure it out when you're deeper into the game
"It's a funny thing, ambition. It can take one to sublime heights or harrowing depths. And sometimes they are one and the same."