Will Surviving Mars be using Steam Workshop for mods, and will that mean that some mods will be unavailable outside steam if I decide to buy the game from GoG or Paradox?
I don't know about GoG, but when you buy from Paradox Store, as well as most of other sellers (greenmangaming, gamesplanet...) you get a steam key, as Magyar1988 already said, so you will have the same access to steam workshop as if you had bought the game in steam ;-)