Pouyou-pouyou: Civ 1 (DOS) -> runs well in dosbox
Civ 1 (Win 3.1) -> hard to run on modern 64-bits OS as it is a 16-bits app (virtualization needed, complicated for GOG to handle and sell)
Civ 2 (Multiplayer Gold Edition / Ultimate Classic Collection) -> can run fine on a 64-bits Windows 7, but needs some patching / tweaking, plus a CPU-throttler ; complicated for GOG to handle and sell
Civ 2 (Test of Time) -> can run fine on a 64-bits Windows 7, needs an unofficial patch and some tweaking ; complicated for GOG to handle and sell

One final thing that may be an unimportant detail : the MGE version of Civ 2 seems to have a slightly different AI from the normal version (not the ToT). Some users might dislike one, so GOG would have to offer both versions to please everyone.

Orkhepaj: Hmm is there any precedent where they offered different versions. Inho they would just stick to one.

morrowslant: Pretty sure that FIraxis doesn't want any competition to Civilization 6, especially from older Civilization games that are more feature complete than Civ6.

Anothername: *checks the Gog store for Civ 6*

... I guess they had enough sales with the Steam release and don't care for another audience.

Orkhepaj: Hmm is there any precedent where they offered different versions. Inho they would just stick to one.

Orkhepaj: enough sales? that's heresy

Anothername: So much enough that they don't want my money. Same as Bethesda and many others. They don't need more money from other stores. The Steam sales are more than enough for them.

Plokite_Wolf: Hardly. 2K Games already released them as part of the Civilization Chronicles compilation in 2006.