I just beat the game without any major issues. There's been two or three freezes but the developer is to blame, not GOG. And the GOG version has also already been patched multiple times (you can see the version on the download page - it's GOG's internal version number so it may differ from the one visible on other stores while it's the same version of the game). And the most recent patch was added yesterday as a separate download, you should have received a notification.

Also the game seems to have its own specific issues. The options screen allows you to deactivate "surfaces" , it may solve some freezes and other issues for people with dated hardware but will result in (even) uglier visuals.

Edit: Okay, it seems that the patch was indeed added later to GOG than to Steam for some reason (still my guess is that the developer provided the patch too late, not that the GOG team was asleep). Anyway, as I said - the thing you have to do is deactivating surfaces in the options menu, apparently this option was added specifically for the stairs/elevator issue.

Yes, since the late patch there is the option to deactivate surfaces and i had no issues with the stairs since then. Only once the game crashed and i had to replay a mission. The game is great, but still amateurs work.