The GNS theory (later recodified as the Big Model) was born at the Forge, a now dead forum mainly managed by the author of the theory, Ron Edwards.
Having been a part not of the Forge but of a somewhat related forum in my own language, I've witnessed part of this story and I've been exposed to the tale of what else happened mainly from the point of view of pro-Forge gamers.
Luckily, a lot of auto-analysis and of responding to the criticisms of users from the opposite culture was going on at the time, which I believe gives me enough insight to detach myself from the Forgite view and provide the most objective panoramic I can.
The GNS theory
The GNS theory at its core identifies three different Creative Agendas, i.e. aesthetic priorities that drive the game as it is played, or in common parlance, whatever drives a player towards playing in a certain way:
The theory states that these three priorities are in contrast with each other.
While they can cohexist and alternate inside the same game session or campaign, each drives the game in a different direction.
Suppose you're playing D&D and you are at 1 hp, hurt and debilitated,
but your opponent is nearly so. Gamism might mean evaluating the
chance of winning and escaping and trying to test your analytical
skills or luck against the scenario, Narrativism might mean doing
whatever brings out the most interesting story development,
Simulationism might mean following the hero trope and trying to strike
anyway because that's what heroes do, or doing nothing but trying to
back away with a limp because you're badly hurt and frightened at the
idea of death (despite the rules imposing nothing of that, and yes,
I've seen this happen in a game I was in.)
Having players push for different agendas makes for an unsatisfactory game where people can even fail to understand what the other players are looking for.
The GNS divide is not meant to categorize players (for one can look for different agendas at different times) or games, but sure a divide is born between more niche rulesets that try to forward a single agenda, thus making sure that every player can choose to participate or not according of how much he likes the agenda, and "you can do anything with this game" rulesets that don't.
Ron Edwards sure saw players who choose this last kind of rulesets as basically shooting themselves in the foot. Such games happen to include the vast majority of the games that existed at that time, from D&D to White Wolf games (that, despite being called "narrative" games, have nothing to do with Narrativism), from Warhammer to basically every big title with a huge fanbase out there.
No wonder games born from the Forge didn't change this. After all games that forward an agenda are necessarily limited in scope (thus more niche) by design.
Games that fail to forward a single agenda are technically called "incoherent games", and this is depicted as a very bad thing because, in the very common event of players without knowledge of this theory trying to play the game differently from the others at their table, it means having fun in turns (ever had the players that get bored during D&D combat and the ones who get bored outside of combat?) and creating a culture of people who learn to accept something avoidable (by playing an "only combat" or an "only diplomacy" game) as a necessary evil.
In a striking fit of
</sarcasm>, some forms of incoherent game play were described by Ron Edwards as "'I learn to accept bad practices in order to continue playing to the point of believing it is normal behavior' mental conditioning," which he further clarified was a form of "brain damage."
Forgite game designers also saw Rule 0 as a necessary patch for a certain kind of games whose mechanics tried to model the physics of the gaming world, and tried to demonstrate that it was possible to build a game that just worked, without the need to tweak the rules.
Followers of the GNS theory saw players who only played incoherent games because they did not recognize what they were doing wrong, trapped in a culture of tribal gaming where the only games they knew were those played by their group of players and even experienced GMs were good at the game only because they managed to endure a long phase of trial and error caused by game designers not being able to tell their GMs what really mattered (manuals tell you how to determine when an enemy dies or what it can do, they do almost nothing to teach you how to frame scenes, create compelling enemies, satisfy the players, ...). In their mind, those players needed help and deserved something better than their old, old-styled games.
To a follower of the Forge (or at least to a large and vocal part of them, since this is what happened and I was among them), those players were fertile soil for evangelization.
It turns out people don't like to be told they've always been wrong and unable to realize.
Other than that, with all their talk of "play a niche game with clearly stated goals, and play it with people who like that game", forgites were menacing an estabilished order made of tightly knit gaming groups.
Non-forgites, in turn, often considered the technical jargon as complex, therefore elitist, and sometimes even offensive (You're calling my favourite game incoherent? You're calling me a Zilchplayer? You're telling me I'm brain damaged?).
Some, who had never encountered the social issues created by the lack of coherence, saw these evangelizers as misguided or worse, trying to fix a problem that never was.
A confusing jargon and an hard to grasp, multifaceted theory (even those who like the theory often fail at grasping some details of it, I'm still there), misrepresentation of games built from the theory1, both communities trying to demonstrate that they were right (and by the extension that the others were wrong) excalated pretty quickly into some sort of religion war.
Religion wars, trust me, are never pleasant.
1- the most blatant example is a rule in Dogs in the Vineyard called "say yes or roll" which tells the GM to have the characters succeed unless there's some active opposition, which very often gets understood as "if nobody's trying to stop me, I can jump to the moon and the rules say you have to let me". This stems from not knowing other rules of the game (e.g. raising an eyebrow to veto some action, a thing every player can do if they think something shouldn't happen in the story) and possibly from trying to ridicule the general "This game works without Rule 0" claim of the Forge people.