zeogold: There's no real set way of knowing whether a game will come to GOG or not. Anything we could tell you would be pure speculation, and on the offchance somebody DOES know something, they'd likely be under an NDA. Your best shot is to contact the devs and ask them to submit their game to GOG.
Edit: You could also create an entry on the Community Wishlist.
https://www.gog.com/wishlist/games I don't quite understand the "lead platform" question. Oxenfree was originally released on both PC and Xbox at the same time. I don't think it was "intended" specifically for one of the two or anything like that.
Contacting the devs about "Afterparty" actually seams like a good idea. It´s a small team. Maybe they will even respond. Something like: "We here your feedback. We can´t talk about this right now." ;)
Regarding "Oxenfree": I know that both versions came out on the same day. What´s interesting is that on release the xbox-version was less stable than the game on pc and also was lacking some bug-fixes which where included in the pc-version right from the start. So this could mean that microsoft delayed a day-one-patch or the pc was the lead platform.
