I'm not saying the game is bad. I do believe that selling "expansions" as in-game items is at the very least a questionable business practice because it can potentially misinform the consumer. However, that is not my point.
I think - and to be clear this is just a hypothesis - this game didn't get picked up by GOG because it's business model is not (or at least was not at the time) supported by this website. The only game on GOG that offers in-game purchases as far as I know is Gwent and, let's be honest, they disabled reviews for it in this websiite for a reason. Gwent embodies everything about contemporary games that people who prefer to buy on GOG are trying to avoid: online only, multiplayer focused, pay to win, etc.
I think The Quest's developers saying the game was deemed too niche by GOG is a way to avoid openly discussing their business model with their community in a way that could put them in a bad light. Nothing inherently wrong with that, but "The Quest is too niche for GOG" is turning into another meme like Jack Keane and a lot people might be getting the wrong idea. Usually, when something doesn't sound like a reasonable explanation, it really isn't.
Redshift wasn't selling in-game expansions when they pitched the game to GOG. This came about much later after GOG rejected them.
They also offer free user-made expansions on Steam Workshop. http://steamcommunity.com/workshop/browse?appid=428880
This is exactly in-line with what they've always done. They offer the editor, users create stuff with it and decide on whether to sell it, give it away for free or keep it to themselves, with no input from Redshift, though I imagine they probably get some cut of the commercial expansions.
Again, Redshift themselves created only the game, the official expansion & the soundtrack. All 3 products have their own pages, are eligible for refunds, and bundled with a discount.
The rest is user created content that adds NOTHING to the main game or expansion, and the only reason to buy them is if you like the game mechanics/artwork enough that you want to keep playing. It is not cut content. It is not cheat items. These do not in any way interact with the main game or official expansion, at all.