gmDoom is a Garry's Mod addon that ports pretty much everything Doom and Doom 2 had to offer, movement is included. This addon is probably one of my favorite addons on the Garry's Mod Workshop but i see alot of confusion on these questions.

1: What is an IWAD!?!?!?!?!
An IWAD or as i'll call them, WAD is a file that came with Doom and Doom 2 that has all the data and assets for the game, This addon DOES NOT come with Doom/Doom 2 Assets therefore you will need a Doom2.WAD file.

2: I can't afford Doom 2 i am poor :(
Doom2.WAD and Doom.WAD are easy to find for free on websites, Some of these WAD files are from the 360 PS3 Doom 3 BFG re-releases so if some of the sprites have been changed that means your WAD file is from one of the re-releases, and not an original WAD file.

3: Where do i put IWAD????????
you copy and paste Doom2.WAD in your garrysmod folder.

4: How do i get to my Garry's Mod folder?
When you go into your steam library, Go to Garry's Mod. Click the settings button, Go to properties, Browse files and it should take you straight to your Garry's Mod folder.

5: Why are some enemies missing :(
The "Missing enemies" like the SS aren't actually missing, they just aren't on the spawn menu.

6: Will my friends see it if they don't have the IWAD?
No they will not, Every player needs a Doom2.WAD file in their garrysmod folder in order to see gmDoom assets.

7: Can this run Doom mods?
I'm not so sure, I know it cannot run anything that will not run on the original DoS release.
Yes that means no Brutal Doom.

8: Wolfenstein3D support?
No

Here is my awesome dumbass guide, Hopefully you find it somewhat useful!