Beginnings: The Witcher 3
Gwent's inception originates from The Witcher 3: Wild Hunt, a 2015 award-winning RPG developed by Polish studio CD Projekt Red. Sometimes called "an RPG simulator in a card game," Gwent started as a minigame that players could play against various merchants. You start with a base deck; winning against NPCs added more cards to your collection.
Gwent became so popular that on June 15, 2016, CD Projekt Red announced a standalone Gwent game.
A Gwent game in progress, taken from the Gwent Wiki (https://witcher.fandom.com/wiki/Gwent).