I am surprised there's no sticky here for brutal Doom. Maybe this one can be.

Brutal Doom is my favourite mod and I would like to show you how to it to work the GOG version of Doom.

1. Download GZDoom http://www.osnanet.de/c.oelckers/gzdoom/download.html and extract it.

2. Download both brutal Doom and Metal Doom and extract the contents http://www.moddb.com/mods/brutal-doom/downloads/brutal-doom-version-20

http://www.moddb.com/mods/brutal-doom/downloads/doom-metal-soundtrack-mod-volume-4

3. Start DZDoom to create a local .ini file.

4. You'll need to add both the Brutal Doom and Metal Doom .pk3 to the GZDoom .ini file under the [Doom.Autoload]

There are two methods of doing this.

Typing in the address of the .pk3 files or adding the files to the GZDoom directory and just typing path=.

Here's an example of method one
[Doom.Autoload]
path=C:\Address\of\brutalv20.pk3
path=C:\Address\of\DoomMetalVol4.wad

Here's what you type if the .pk3 files are in the dirctory
[Doom.Autoload]
path=brutalv20.pk3
path=DoomMetalVol4.wad

5. Next GZDoom will have to find the Doom.wad file. It should be in C:\GOG Games\DOOM

Doom II C:\GOG Games\DOOM 2\doom2

Final Doom TNT C:\GOG Games\Final DOOM\TNT

Final Doom Plutonia C:\GOG Games\Final DOOM\Plutonia

The easiest way to make GZDoom find these files is to simply copy them into the directory, but you can also add their address to the .ini file if you want.
If you want to add them to the .ini instead of copying them follow this example
[IWADSearch.Directories]
path=C:\GOG Games\DOOM
path=C:\GOG Games\DOOM 2\doom2
path=C:\GOG Games\Final DOOM\Plutonia
path=C:\GOG Games\Final DOOM\TNT

That's it, I hope this was helpful and let me know what you think of the Brutal Doom mod.