Again the friction has nothing to do with the fact it's a gacha. It being a gacha is just ammunition.
Fate has had some "normie invasion" events before, but none of them even compare remotely to what FGO brought along, IIRC in one year it became the entry point for most fans of the franchise. That dwarfs anything any anime managed to do previously.
That influx will invariably change the discourse of the fandom in general, some ppl will miss the way it was before.
That influx will invariably change the company's attitude regarding its products, some ppl will not take kindly to the course correction. And they might hold a grduge because of it.
FGO's enormous success was initially a surprise to everyone involved. But from then on it took over everything in the franchise. If you were waiting for something not related to FGO, or Fate spinoff anime #40678, I think it's very understandable you get a chip on your shoulder when that's literally everything that's been coming out for the last 6 or so years.
None of this has anything to do with it being a gacha. It's all about how it took over the entirety of TM by storm and making it laser-focused on appeasing FGO fans only. The fact that it is a gacha might add fuel to the fire, as it certainly did for me, but the source is this feeling of being left out, of being unheard, when we were here first.