Being an ex-Unity dev too, I can fully understand your frustration with the Gameplay Framework. Feels like in Unity you have this clean blank canvas to start on, but in UE4 you are knee-deep in clutter, constraints, and pre-made stuff you don’t want in your project in the first place. You can’t really “disable” it, but you can ignore it almost completely (unless you’re making an online game. Then you’ll be happy you have it.)
You’ll have to handle input in the PlayerController. But aside from that, just make your own Managers and whatnot instead of using all this GameMode/GameState/PlayerState stuff. The default ones are harmless and won’t do anything in particular by themselves
There may be a way to make none of it appear if you create a custom GameMode and override some of its initialization functions (GameMode::InitGame(), maybe?). You’ll have to look into it
It is possible to become comfortable with UE4’s sea of polluted code and all the stuff that it imposes on you and forces you to live with, though. It just takes a lot of time and effort. I guess we have yet to see a “perfect” engine getting released. It would have to have Unity’s “blank canvasness” and UE4’s rendering/tools quality, which is by all means a realistic goal