It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

It appears we have yet another developer which treats GOG users as second-rate citizens. The GOG version of this game is Update 1.3 (01 June 2018). Steam, on the other hand, has been given Update v1.4 (25 June 2018), which fixes "Game crashing in Chapter 9" and other things, and Update v1.5 (26 June 2018), which fixes "Chapter Select Bug" and other things.

I guess this shouldn't surprise me, considering Toxic Games' website shows Steam only, no mention of GOG anywhere, and the "Buy Q.U.B.E. 2" link brings you directly to the Steam product page. Also, the trailer shown on Toxic Games' website shows Steam, but not GOG (trailer shows "XBOX ONE Windows 10 STEAM PS4").

Why bother releasing the game on GOG if you have no intention of maintaining support? This makes me seriously question whether the as-yet-undelivered Season Pass content will be provided as promised, especially considering the Season Pass product page on GOG still shows "Classic themed DLC puzzle pack 1 - 10+ New Puzzles (Out Q3!)", without any additional information, but on Steam, the developer posted the puzzle pack 1 (a.k.a. "Lost Orbit") details on 11 July 2018.
I used the contact form on their site in order to remind them of our existence. I encourage others to do so too.
We’re working on an update for GOG but I cannot confirm a release date at the moment.
We will have continued support as well as bringing our DLC there too.

So they seem to be working on it.