As far as physical card games go, Gwent is on the low end of things when it comes to complexity and calculations. Magic the gathering exists after all.
I haven't played MTG in a few years (and don't intend to come back what with the SpongeBob crossover and all) but Magic used to at least try to limit mental math in terms of changing values on cards. Buffs lasted a turn, and anything permanent was auras or equipment or +1/+1 counters.
Gwent has a lot of numbers changing value contextually, often by multiplication instead of simple addition. Now, combat math and all that is way more complex in Magic, but Gwent does have lots of changing numbers to track.
I am getting back into MTG currently after a nearly twenty year lapse. I do like much of how it's evolved, but every fucking card is a novel now. The math isn't ever that bad, but it feels like you need a notebook to understand all the cards on the battlefield during a game.