yogsloth: I'll tell you the answer when I find out why Gemcraft was rejected.
I'd imagine that most of the GOG staff are also game testers. Hopefully each game is overviewed by multiple people, and not just a single person for multiple games. They go through each submission and play the game for several hours. If the game is engaging, and bug free, the submission would then be considered by other people, who study the potential monetary value of each game. I'm certain there's a more descriptive term than that, but my vocabulary fails me now.
If it just so happens that the game doesn't resonate with the people who tested it, or has bugs, or is considered too niche for the GOG audience, I think would result in a too generic, yet still harsh, response: "the game didn't match their requirements for selection" "the game is too niche for GOG". I said it before, I think GOG needs to work on their communication skills when they refuse a submission. Human brain is wired to feel bad about failure and rejection, to also feel unjustly dismissed only makes one angry/bitter.
I hate it that I keep reading about games being rejected, and the developers expressing less than amicable thoughts about their interaction with GOG. I think I'm becoming jaded about this.
EDIT: About Gemcraft, I played the game on Kongregate(when it first came out), I got it on STEAM(from one of those cheap Fanatical bundles), and I would most likely get it here too. The problem is that I think I would only replay it if had Galaxy achievements. That would be the only thing that would get me to replay it.
About Attentat 1942, that is not a game I'd play, but I believe it would have its public, if release on GOG.