I don't GG when the game doesn't feel fun for me (or) if I'm just not having a good time for any reason unrelated to the other player.
Gameplay that destroys my strategy, basically disabling my ability to play or do anything isn't fun.
Examples of things that don't feel fun:
-lock all my cards, and/or my leader abilities, and/or who siege and use all my cards against me (worse if they do ALL of these). This makes basically the round unplayable for me, esp. if the rest of my cards don't work against this specific faction. (I sometimes just straight up forfeit because it's that unenjoyable for me)
-playing the the same faction as me (with rare exceptions), ex. if by playing them I learned something new about my faction I didn't know before, then I GG.
-whose deck is simply better and overpowering than mine i.e. we have an imbalanced deck (if I didn't stand a chance, then it's simply not fun)
The majority of people I don't GG are Nilfgaard and Syndicate, Northern Realms is also included in that list but I don't see them nearly as much as I do Nilfgaard and Syndicate to remember what they do that I don't like. It's not fun to play against them most of the time. And when I get around to playing those factions for the achievements it's going to feel really dirty.
Also sometimes I don't GG because:
-In the beginning I didn't know what GG was or how to do it (there wasn't a tutorial on GG sportsmanship). I thought the game was just giving me GG not the player.
-I'm just having a bad day and overwhelmed juggling my family and responsibilities, sometimes I don't remember to GG or just am not feeling good/having fun in general.
-I'm burnt out and not enjoying the game because I played too long trying 'to grind', and neglected my other duties on that day.
Please note, I don't actually mind losing and learning from my loss, but for the most part I don't GG when it feels unfair, like I didn't even have a chance, where no amount of 'good strategy/gameplay' would have allowed me to win, short of changing my deck entirely to cards I don't have and have to 'grind' to get. (Something I can't even do until the auto-mill problem is fixed).