Not really sure, even if its been 6 months ago, but in the GarrysMod/garrysmod/data
pathway, there's the horde file that is the horde gamemode files and it also contains the
ranks file and the text documents for the class, enemies, items, perk_choices, and the tips.
So you could figure out a way to get a MW2 only config, but you actually have to
track what you put in, so you don't get an error with the scripts.
You don't have to mess with the map files.

you are very late and you can just do that in game. You don't need to go in the addon files.

Yep, though you can just copy the documents and store the document somewhere in case
you want different items and enemies for different themes and difficulty settings.