In case anyone is wondering: v1.042 isn't a new update that's exclusive to the Steam version. You can easily update the GOG version to v1.042 by using the Unofficial Patch mentioned in this post.
But please note that if you patch the game to v1.042, all your current Single Player saved games will no longer work.
If you try to load them, the game will crash. You need to start over from the beginning after patching.
I'm guessing the folks at GOG felt v1.030 was more stable in Single Player than v1.042. Which may be true. But at the same time, v1.042 also fixed some additional issues in Single Player. Most notably, it corrects mistakes that prevent you from reaching the correct final build point score when you finish the campaign.
v1.042 also added a whole new mode to Multiplayer. (Which you can still play. There are multiplayer servers run by fans, and patching/modding the game will allow you to play on those servers. Again, mentioned in the post I linked to above.)
And I'm continuing work on v1.2 of the Killer App Mod, which will bring a lot more Single Player fixes to the table:
Single Player Fixes 1/3: https://www.youtube.com/watch?v=Eor2DXBXQRQ
Single Player Fixes 2/3: https://www.youtube.com/watch?v=hVakVVLDTC8
Single Player Fixes 3/3: To be added (pending completion)
It's unfortunate that GOG hasn't implemented the latest patch, since users who later discover and apply the patch will have to start over in Single Player, which would have been preventable.