I'm not quite sure what the situation with this game... I mean the old DOS version. There is the new one and the redux version, but the classic version is free on Steam and the source code was released AFAIK... shouldn't be free here too?

There seem to be several versions of the game:

The original one
Classical version - original game + two expansion
Classical redux version - same as above, but with additional graphic upgrades and ported into Mac and Linux

What Steam offer for free is the first one, the third one is still sold at a full price, the GOG version is the second one.