I've really enjoyed Gwent. Best mini-game I've played in a long, long time. It becomes a bit too easy though once you figure out some of the tricks and upgrade your deck a bit. I'm using an upgraded Northern Realms deck and have won probably 18 of the last 20 games I played.
You want to set up your deck to maximize the number of cards that you'll be able to use. Spy cards are the best, they give your opponent points but you draw 2 new cards when you play them. Revival cards are good too, they let you pull a card from your discard pile and re-play it. Decoys are also good to have, because if the opponent pulls out spies, you can grab them and use them yourself (or reuse the revival card to pull a second card from discard).
I'm fond of baiting opponents into getting over confident and passing in the first round. In my ideal game, I'll draw 2 spy cards, giving the opponent about 9 free points but leaving me with 12 cards after I've played them. Often then if the opponent has 25 points or so while I still have < 10, they'll go ahead and pass. But if I have the right cards I can generate 30-40 points with just a few cards (2x Blue Stripe Commandos + Commander's Horn = 32 points). So I beat them with maybe 8 cards left, while they're down to maybe 6 cards. Then I draw an additional card with the Northern Realms faction perk. If they get way ahead though, its well worth passing and losing the first round. I've had some opponents (monster decks especially) throw out 50-75 points or more on the first round. If you pass and lose just remember that you still have to beat them twice.
It's also good to know how the decks usually play, and stack your decks appropriately if an opponent is giving you trouble. For example monster decks can be very intimidating to play against but they're basically all close combat, so you can shut them down easy with a frost card.
Anyway, I didn't mean to write a whole essay on gwent.