Next to 2.1 it says (GOG-16)... How long had this game been on GOG before actually being released for public purchase? As early as the Steam version?
I've noticed with many releases here, that they are not the first GOG upload. Some will say GOG-2... others even GOG-4.
Any apologists wanna help me out here?

The GOG.com release only happened when Patch 2.0 was released, and that patch was only recently released (ie a month or so ago), hence the large number you are seeing in-game. It has been on Steam ever since its original release in late February 2015 and, up until recently, was the only place you could get the game.

That's not what I mean. A Twitch streamer even said that they get access to GOG games way before the public do.
If you bothered to notice, whenever a new version of GOG game is uploaded, the (GOG-x) will move up to the next number. A lot of the time, games will release at GOG-1; however, some occasions this is not the case.

Well given that this game has never been on GOG.com until just recently, that statement makes no sense. In fact, GBX went to great lengths not to make any confirmations about the game going on GOG.com until they tweeted when it was already released on GOG.com

Also, I don't "have" the GOG.com version, because I bought the collection on release at Steam. If you actually read my post properly, I said that this game has been around for a while, so 16 probably refers to the 16th code build of the game.