Gwent: What exactly can I gain or lose?

If I play the card game 'Gwent' in the beginning of the game (Lvl 3 or 4), what exactly can I gain or lose?

Will I gain or lose money?
Do I have to pay for a lost round or game?
Will I a lose one of my cards?

What is the impact on the further story?

There are 2 types of games. Quest games and normal games.

Normal Games usually start out by you betting a certain amount from 1 to 10.

If you bet 5 and lose you lose 5 Crowns. If you win you gain your 5 Crowns back plus 5 from the opponent.

Quest Games usually are opponents whom you play during Gwent quests and they don't bet anything. They just stake an unique card of theirs. And if you win you gain that specific card. If you lose you just lose the game basically, you don't lose card or Orens.

There is one case however that some people make a huge mistake.

The Gwent Tournament in Novigrad seems just like a Quest where you play a tournament of Gwent.

If you actually win every game you will be facing a guy in a fist fight.

Some people make the mistake of going to this quest at low level like 15 and have to fight a lvl 27. It's a huge mistake and I'm gonna give you this heads-up.

So basically summing it all up discounting the Tournament Quest you have no real reason of fearing to lose Gwent games. Only if you are really low on money. When I came into Novigrad I started playing barkeeps and Merchants and I lost like 10 games in 15. Just practice to the Gwent quests and get the Unique Cards and you will be unbeatable in no time.

As far as I know there is no big impact on the storyline.

At the beginning of most games you can bet your money and if you lose you will lose all the money you bet, if you win you will get double of the money you bet.

You can never lose cards, you will only gain cards by beating opponents. Some opponents will say they like only to play on high stakes but it is only them who take the risk since you will never lose cards.

Will I gain or lose money?

Yes. As stated by Lokuzt above, in the scenario "if both players bet 5" you would win a net 10. If you had lost, you would have just lost your 5.

Do I have to pay for a lost round or game?

Yes. You have to pay whatever coin amount you bet.

Will I a lose one of my cards?

Nope. If you're playing someone for the first time you'll gain a card (if you win), but you'll never lose any.