I understand what you're saying and I'm sure GOG wants to have the games on their list as soon as possible (preferably during release day).
I'm not sure how GOG does its publishing, but I'm under the impression that developers give their files to GOG's staff and they make the installation package. Then they have to test that the package works with different operating systems. This takes time (no idea how long), and I can imagine a situation where a developer polishes its game until the very last moment. Like in D:OS's case let's say they do changes until the noon on 30th June. When the changes are made they can just publish it on Steam, but have to send the files to GOG. This causes the difference in release time, but who is to blame? I'd say no one, because the developer wanted to make sure that their game is as good as possible on release. And GOG didn't have their hands on the files before this and still has to do all the packaging and testing.
Of course if I'm wrong about how GOG does its publishing, disregard that whole chapter ^^. What I'm trying to say is that there are always circumstances that we are not aware of. For that I don't think we should look for someone to blame.
