Regarding mods, it's a bit complicated.
Steam workshop mods can't be downloaded directly without the game on Steam.
That said, for GOG-released games, mods are sometimes available on third party sites.
By the descriptive text provided when opening the mod manager or mod editor in the GOG release of Surviving Mars, I get the impression that there may be plans to create a mod distribution service separate from the Steam Workshop.
I could be wrong there, but that's the tone I get from the wording.
It may be possible to find websites that allow you to download Steam Workshop mods without actually having the specified game on Steam, but I do not know of any that are currently functional.
Regarding patches, as mentioned, CD Projekt tend to take time in testing patches.
From what I understand, having learned a bit about the process while waiting for The Long Dark updates, this is to reduce bandwidth costs on their end, while ensuring a working patch for the users.
It takes time, unfortunately, as the CD Projekt team isn't particularly large, and they do have a relatively large number of things to do each day.
It can sometimes take up to two weeks for patch to be released publicly.
In regards to GOG versus Steam...
Specifically speaking of Paradox and Surviving Mars in this case.
Paradox has stated that all games they release will be DRM-free; even the Steam releases.
If they have kept their word, then the Steam release of Surviving Mars should be DRM-free once you have started it once.
It doesn't come with a nice installer like the GOG release, but it should work nonetheless.
(I don't mind the wait on GOG too much. This is partly due to my situation though. There is no internet access available where I live, and I must drive 20km to town to use free WiFi. Wouldn't be too bad if I had a relatively good laptop. Unfortunately, I only have a cheap Dell Latitude 2120 netbook. I literally cannot even play Gwent on this thing.)