Lexor: Why do I have to download and store(!) whole G2 data two times if I want to have access to both G2 alone and to G2+DLC?
The way i understand it with a Full Conversion Mod, is that it isn't the game plus DLC, it is the game given a major makeover, so any game files in it are required for the mod to work. So while technically you could say you are getting the game files twice if you own the game and the mod, in reality the game files are modified in the mod, so not quite the same.
A full conversion mod is quite different to a regular DLC. Normally a DLC is an addition (new level, new quest, etc) to a game or a minor makeover (skin, new weapon, etc). While a full conversion mod is a full major makeover, essentially creating a new game.
Apologies for my part in going off-topic. I don't think I have anything more to add. Blame GOG for creating the issue in the first place, and some of us then trying to clarify things.