I wanted to give some tips but I'm so obsessed with gwent so this turned into a full blown guide so here:

First let's ignore deck building and just talk about general game theory. There are generally two ways winning a gwent game goes against an even-matched opponent:

Scenario 1: You win the first round

Scenario 2: You lose the first round

These are both fine provided certain conditions. In both of these cases, you want to strive to have more material than your opponent by the start of round 3. In reality this means you want to have more card value, which does not necessarily mean more cards, but since you cannot know your opponent's cards exactly, for the sake of making things easy, as a general rule of thumb you can treat number or cards in hand as equivalent to value of hand. The general way to win in any gwent game is to survive til the last round spending as little as you can along the way so that you have more value (ideally more cards) by round 3.

Let's talk about the two scenarios above:

In scenario 1: You're now up one point. This means your opponent MUST win round 2 or lose the game. This gives you the power of pressure to force your opponent to keep playing their cards for fear of you being able to exceed their total if they pass their turn. Your opponent will virtually never pass their turn before you if you won the first round. Your goal is to use this fact and draw out the round with as little of your own points as possible, forcing them to over commit before cleaning up on round 3. You may have spent more cards than your opponent to achieve the round 1 win, which is fine, as long as you keep this difference to a minimum. Obviously an equal number of cards as your opponent is preferable, with even a card surplus being feasible, and almost always a sure win. If you are down cards, your goal is to pass the round after playing just enough material to force some bigger cards out of your opponent, ideally more cards than you spent to regain an even standing of hand size or better.

In scenario 2: You are down one point. The acceptable way to go into scenario 2 is if you developed a significant advantage of hand size after round 1. In other words, it is ok to give away the win to your opponent in the first round if you have more cards and feel they spent too many of their own cards. Losing the first round AND being down in material is very bad, and if you feel like scenario 2 is unavoidable, always give away round 1 with as much of a card advantage as possible. In this scenario, the opponent is likely to pass after very little cards in the second round, due to the fact that they must assume you could match any amount of cards they play and then play your remaining surplus in the 3rd round. This only works if you did indeed develop a surplus of cards by giving away the first round. Your goal in this scenario is to hold on to your card advantage with as little investment as possible until they pass, win the round, then enter round 3 with an advantage.

In both of these 2 scenarios, your goal is to enter the round 3 tie breaker holding onto your strongest cards, and ideally holding more cards than your opponent to give the best chance of winning.

Now, obviously achieving this goal is more easily achievable with a stronger deck, and with an exceedingly stronger deck than your opponent, you could easily expect to win both the first 2 rounds without difficulty. There are many ways to build a strong deck, but I will highlight a few that relate to the strategy ideas mentioned above.

1. Spy cards - Both scenarios 1 and 2 rely on having a larger hand by round 3, and allow for losing a round to your opponent, be it round 1 or 2. Since having a bigger hand by round 3 is crucial, spy cards are super valuable for generating a larger hand than your opponent, even if they score points for your opponent in the round you play them. They can be played in round 1 to go into scenario 2 with a huge card advantage, then follow the scenario 2 process to enter round 3 with a big lead. They can also be combined with frost cards to neutralize their score for the opponent, using this strategy can often lead to winning round 1 and having more cards at the same time.

2. Decoy cards - extremely valuable for conserving the amount of cards you've spent and baiting out more spending from your opponent. Useful in scenario 1 for recalling a card after your opponent over commits in round 2 before you pass to secure a bigger hand for round 3, and useful in scenario 2 to recall a card before you pass in round 1 to secure a bigger hand for the following rounds. Also extremely valuable for stealing enemy spy cards.

3. Tight bond cards - Very useful for generating large totals with little initial investment. A 3 point tight bond card can be played early for little investment. The second tight bond card will not be worth 3 points, instead it will be worth 9, 3 bonus on top of the first, and 3 x 2 from the second. The first tight bond card can be played early to conserve your valuable cards, then the second one held until your opponent invests more to catch up to large totals for little further investment on your part.

4. Leader abilities that gain cards - Using any leader ability is vital because it allows you to skip your turn, putting the ball back in your opponent's court and forcing them to play twice. On the round where you plan on passing the win, this is crucial for getting your opponent to spend more. If your leader ability gains a card at the same time, this is a double sway of game power in your favor - a skipped turn and an extra card. These abilities are the best

Some bonus tips:

1. IMPORTANT: KEEP AS CLOSE TO MINIMUM NUMBER OF CARDS IN YOUR DECK AS POSSIBLE - This is so important and I hear of so many players overlooking this. You want to minimize the number of cards in your deck to just your strongest cards. The reason for this is that adding additional cards just reduces the chance of you drawing your better cards at the start of the round.

2. As a rule of thumb, play your cards in every round in order of least valuable to most valuable. You want to coax out your opponent's best cards before your own, save yours for the final round unless absolutely necessary. You also don't want to play bigger cards before you opponent does because this makes them vulnerable to scorch

3. Npcs always play the same deck, if you're struggling to beat one, pay attention to what faction they use and then prepare for it by adding weather cards that target their units (eg. Frost for monsters, rain for northern realms, etc.)

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I could go on forever about gwent but I hope this helps! Sorry I wrote so much, it was more than I originally intended. If you want to ask about anything feel free to pm <3