This question got me wondering.
Does the difficulty you play on have any effect on the difficulty of gwent or other mini-games?
I don't have an official source for it, but i played the game on two difficulties (bst and now death march) and the cpu doesn't seem to use other tactics or different cards.
Also, in previous Witcher games, minigames (Dice poker or arm wrestling) were unaffected by the difficulty of the game since every opponent already had his own difficulty level. I think this is still the case in Witcher 3.
The current answer is outdated. The new patches have added a Gwent difficulty that is independent from the difficulty of the rest of the game.