On Gwent, which has really grown on me since I started off playing the game:
Just an FYI...if you intend to try to get all the Gwent cards, I'd recommend starting right away early in the game. Every time you talk to a merchant, buy all the cards they offer. I absolutely hated Gwent at first, and the difficulty curve is absurd for it, as you'll get your ass handed to you constantly early on by people with much better hands than you. Remember to play merchants too. You get a random card from every generic NPC you play with and special cards from the quests.
Apparently they are patching in (might have done it already) a fix that makes sure you can get cards you missed out on from characters who disappear from the world, turn hostile to you (like the innkeeper at the White Orchard inn from the start of the game) or are killed. I know for a fact you can get the Bloody Baron's card without playing him, as I got it from his desk after he departs from the game after his Family Matters quest. I'm also pretty sure the cards that above mentioned innkeeper has in stock are transferred to the nearby merchant who sets up shop after you save him from the griffin attack at the start of the game.
As far as strategy, manage the fuck out of your decks. You only need 22 cards to have a complete deck, so start tossing out your shitty ones as you get above that number. There's no need to have guys with attack powers of one or two unless they have some sort of nice special ability (like medic, spy, rally, or what have you), and keeping your deck as light as possible gives you more of a chance to draw your elite cards that can't be eliminated or depowered by scorch or weather effects. Also, I've found that weather cards are mostly useless in high level Gwent. They're pretty crucial early on, but once you get a better deck I wouldn't waste card slots on them.
Scorch and the Commander's Horn are your friends, as is the Dandelion card (which does the same thing as the Commander's Horn) you can get from the ball you attend with Triss, so make sure you do that optional Gwent tournament during that quest while you can, and certainly spend enough time crafting a solid deck before you do or you won't be able to win. The Gwent tournament in Novigrad is also ridiculously hard. It took me about five tries and switching from the Northern Realm to Monsters to win the second match (although I won the other three on the first try), as that bitch has an incredible deck and can abuse most early game tactics with ease. And though you may be tempted, do not sleep with the chick during that quest. I've read she steals some of your cards if you do.
I'd personally recommend doing the Gwent-related quests as early as you can manage them. Before you do, though, visit a bunch of merchants and stock up on their cards in addition to playing them. That's a great way to build your deck well enough for the Velen quest, which is tough (the soothsayer in particular will fuck you up) but not impossible.
Here's my personal strategy for early game Gwent play:
1. Use the Northern Realms...you basically have to early on because it's the only one you'll have enough cards for.
2. Use the second Foltest leader card. It removes all weather effects on the board, and early game Gwent involves a lot of weather effects.
3. When an enemy uses a weather effect, make sure you keep track of how many points you'll have when you remove them. This is important for obvious reasons.
4. Never use the Foltest leader card right away. You always want to wait until your opponent passes, as they might have another weather card and just fuck you over immediately after you waste your leader card. That's why you keep track of how many points your cards are actually worth.
5. Always start off playing lower value cards. If you're lucky, your opponent will commit hard to a round and play one of their hero cards, at which point you can either decide to try to win the round (if the hero card is weak) or use low value cards to make your enemy waste a few more cards before forfeiting the round.
6. Spies (you place the card on your opponent's side and then get to draw two cards from your deck) are your friend, but they are best used when your opponent has already passed on a round. If they haven't, there are good odds the fucker will use a decoy and then deploy the spy against you immediately. They can still do that with a medic card, but I feel like those are more rare early on than decoys. Everyone has a goddamn decoy.
7. Decoys are also your friend. If your opponent plays a spy, steal it with a decoy and feed it back to them to keep the playing field level.
8. No weather effect cards. None. At all. Clear weather is acceptable, but I'd only recommend keeping one in your deck.
9. I forget what they are called, but some of the early game cards have the ability to double in strength if you place them next to another identical card. These can be really valuable. However, if you only draw one of them at the start of the game, it's usually better to use your swap on it and hope you draw a better card.
10. Use your two swaps wisely, and be smart about not using them at all if you draw a great hand. There's really not much reason to have more than two Commander Horns, for example, if the majority of your cards are all one type (like Siege or Melee).
After I switched to the above strategy, I didn't lose a match for the longest time until I entered the big Gwent tourney in Novigrad, and that quest contains the toughest matches in the game (you also can't save between matches two and three, which is still bullshit even though it didn't impact me). I had to switch to my Monsters deck to win that one, and Monsters are insanely overpowered because of those fucking gang up cards which will absolutely kick your ass repeatedly early on. If anyone is interested in my Monsters strategy, I'll share it. Otherwise, the Foltest one above should help any of you who are getting frustrated by early game Gwent.
The game is actually really fun once you get hands good enough to actually win on strategy instead of luck.