if anyone's having trouble installing and iwad such as freedoom then here's the guide on how to do so:
1.install the IWAD of whatever doom retexture you wish to install, such as freedoom
2.open up the folder/winrar and keep it open for now
3.open up the files for garrys mod as instructed in the description for the gmdoom page
4.when in the "garrysmod" file (its inside the garrys mod steam app files), paste the iwad into the folder, the iwad should only show the name
5.rename the iwad "doom" or "doom2" depending on what iwad it's based on (a doom 2 remake iwad is recommended again)
6.instillation complete
note: if the addon shows black and pink boxes or original doom textures, then the iwad was not installed correctly, or another iwad is conflicting it. other iwads such as heretic, or hexen will not work since it's not just a retexture of doom. something like freedoom phase 2 will work beautifully!
q&a
q: some enemies are invisible whats wrong?
a: this means the iwad doesn't use textures for that enemy, you'll have to do one of who things 1. change iwads 2. deal with it
q: why am i still getting doom sounds?
a: i can't fix this you'll have to put up with it