Is Gwent a good way of making money?

No, you can't wager more than 10.

If I remember correctly, some NPC that can play gwent have a "high stakes" type of betting schema where you may potentially win more money. Usually these types of encounters happen via completing gwent quests.

Once your deck gets big enough and your level is high enough (like lvl ~25), you can compete in the "High Stakes" tournament (it's a quest) and win a lot of money... after you fight a group of thugs and other competitors. So yeah, don't do it before level 19 or 20.

As a side note, I think the best way to win money in Witcher 3 is to find treasure chests and complete witcher contracts.

Edit: Someone else pointed out that you have to pay 1000 up front to enter the High Stakes tournament. Absolutely true. You can't do it if you don't have the $$$.

If you have blood and wine many npc's will bet for 50 which can net you 100 every win, not the best method but its consistant and reliable