So is it full episode one of game that will have more episodes? Or is it demo of episode one? Year ago I have seen prolog demo of this game. Will it be included too, or was it cut?

Looks like there are three episodes included already and Early Access is just there to polish things further. It was also mentioned that the demo is some kind of prologue to episode 1. Would be nice to have it on GOG too as it's not included in the Early Access version (yet).

I am very curious about that game. It got very interesting art style and animation sometimes look outstanding. But I have also play many Doom mods and I know that there are stuff I like and stuff I dislike (slaughter maps, only pistol start per each map/episode, levels based on combat arenas rather then exploring). Also it looks like episodic release so there is a risk it can be left at unfinished state or drastically change gameplay quality between episodes.

If developers create more content earlier why not include it? I mean it is a part of story / game and it is not free game, it cost real money, but have half of content cut - that is what I feel.
Also there are some rpg elements, does guns I collect and upgrades goes with me to next episode or each episode is fresh start from nothing.