Ok, so apparently the workshop addon of Zombie Survival is outdated. That is why it is broken.
If you would like to continue playing Zombie Survival alone for some reason and are willing enough to go through the trouble to PLAY alone, follow these steps:

1. First of all, download the most recent and fixed version of Zombie Survival through JetBoom's github page: https://github.com/jetboom/zombiesurvival
Link is also in the addon's description.

2. Make a folder, anywhere, and just name it "zombiesurvival" for example.

3. Get the zombiesurvival-master.zip you just downloaded, and put it into the folder.

4. Extract the contents of the .zip into the folder. You should have the zombiesurvival-master.zip and a folder named "zombiesurvival-master". Delete the .zip as you won't be needing it anymore.

5. Now, go to where your Gmod files are located. For example, I have Gmod installed in my second drive, so I'm going to go to this directory:

D:\SteamLibrary\steamapps\common\GarrysMod (keep in mind this is what a steam directory looks like when it's not in the Main drive, so don't get confused)

I'm not going to write every single possible directory Gmod could be in, so I'm just going to assume you guys aren't that mentally challenged and can find it by yourselves.

6. When you are in the "GarrysMod" folder, there should be another folder also named "garrysmod", no capitals. Open that.

7. in "garrysmod" open the folder named "gamemodes"

8. Now go back to the folder you've just created and copy/cut the "zombiesurvival-master" folder. NOT THE ZIP (WHICH YOU AREN'T SUPPOSED TO STILL HAVE ANYWAYS)

9. Paste that folder into "gamemodes"

Installation is done, now I will show you how to play Zombie Survival.

10. When Gmod is fully opened, open the console and type:

gamemode zombiesurvival

11. In order to run a map, type this command:

map [name-of-ur-zs-map]


Ok, that's basically everything. Leave questions in the comments and I will try and answer them.