Nuclear Dawn is built on Left 4 Dead 2's engine branch afaik.

So this custom config is mostly meant for Garry's Mod mapping, am I right?

I assume I can do the same by editing gameinfo.txt file for the game I'm making map for, isn't it?
I already always edit gameinfo.txt to add my projects' paths.

But anyway, how can you ship your map then when you have a lot of custom content used in it? E.g. if I make a map for Nuclear Dawn and use HL2's assets, then I give this map to someone else, he will have purple textures and ERROR models.

What does this custom config feature allows us to do?