"GWENT: The Witcher Card Game" is a canon? The game takes place in the game universe?

It's exclusive to The Witcher 3 game by CD Projekt Red. Does that make it canon? As far as the books go, no, but canon is not exclusive to original material. We shall see.

Would be more challenging if each AI opponent, had a random set of decks, from more than 1 faction, instead of one predictable deck. Bet 1, learn opponents faction, bet 20, play best deck vs him. Always win strategy.

The funniest part is that Gwent was developed a couple months, or even weeks before the release of Witcher 3.
I was listening to some audio books of the Witcher novels. And there was only the dice game (Dice Poker) which Geralt and every one else in the novels played. (Game wise) Dice poker was only in Witcher, and Witcher 2.

In the first Witcher, it was required to play Dice, (especially on harder difficulties).
In the second game, it was not much required, because the reward was quite small. But the dice was now controlled through the moment of the mouse, or console. While in the first game, the dices were rolled by the computer.

Yes it isn't canon to the book.
But is still a very interesting and welcome addition to the game.

You read something different I'm afraid. I can't recall dice poker being explicitly mentioned (but it has been some time since I read the books, so maybe it's there somewhere), whereas there for sure was card game called Gwint. It was probably in a story that had Villentretenmerth in it (Granica Możliwości - The Bounds of Reason) - the dwarves were playing it at the campfire. It was vastly different from The Witcher 3 mini-game, but not much was stated about it. From description it could be inferred that it's something akin to real-life bridge (or whist), because it had bidding at the start, and you had to bid the number of tricks you intend to win, and there were four players.

Edit: fixed typo