First of all: I like the "no-pressure" attitude of your game.
However - it's also a real niche game - it needs a very special kind of customer, who finds interest into it.
GOG is a business, and as such they have to evaluate if a new product in the store will be a worthwhile addition (meaning: a financial success).
And as it looks now, the game has only 61 owners on Steam (of which at least 5 got the game for free), despite being released six weeks ago, and being on sale right now for 15% off.
Add to that an already low base price of €3,99 and it would probably cost GOG more (lawyers fees, etc.) to bring the game into its store, than it would make them money in sales.
Let's say (just for the sake of it), the same number of GOG users (61) would buy the game in the first six weeks after a GOG release for the full price - that's only €243,39 gross revenue.
Or €73,02 net revenue for GOG (= the money, that GOG actually gets...and that's pre taxes!).
That's simply not financially sensible.
Edit: typo, missing word