I tried getting it to load after V20b in the ini file and run V20b like the description asks, but it doesn't work at all. I know this because I am still getting those fucking awful Evil Marines in the map Grosse, which V20c is meant to fix. When I try to load the PK3 directly, I get an error message " 1 errors while parsing DECORATE scripts" and I don't know how or if I am able to fix it. Easily anyway.

The error is screenshot in the image attached. And yes, when running V20b I am using a new savegame and it still didn't work.

Brutal Doom v20c is an unofficial addon to Brutal Doom v20b (and v21 will soon be released anyway), so I've not used v20c myself. The errors in your screenshot are just warnings, though, about references to classes that haven't been defined, so my first thought would be the load order, and is v20c being loaded before or after v20b (it needs to be loaded after v20b).

I listed it as after V20b in the PK3 and WAD load order in the ini file i.e. I wrote the path for V20c just below the path for V20b. Maybe it's trying to load them out of order despite the paths being arranged correctly?

Why are you loading it in all of the autoload sections? If you are doing it for all versions of Doom (both Doom 1 and Doom 2) then you only need to put it in the doom.Autoload section. By calling it in multiple autoload sections it will be loading the files multiple times (i.e. if playing Doom 2 then it will carry out the global autoload, then the Doom autoload, then the Doom 2 autoload and finally the Doom2 commercial autoload, so four times) which may be the cause of the issue here.

Also, putting it in the Global.Autoload section will cause issues if you load any IWADs that don't support Brutal Doom (assuming that you use just the one ini file for all games).

You can always check the load order by looking at the console output when GZDoom loads.