The issue you're running into is stated in the description.

If you're having trouble seeing it in game, make sure you do not have any M9k-Base sweps extracted in your addons folder.

or

If you're no-longer seeing the addon once you've subscribed, extract the .gma file into your addons folder.

To do so you'll need to use GMad Extractor: https://www.moddb.com/downloads/gmad-extractor
Put the .gma of the addon above into GMad, extract the files within the addon, put the files into a folder inside of your addons folder found here.
>Steam\steamapps\common\GarrysMod\garrysmod\addons

Ill try some fixes

Huh weird isnt working for me

No, it's a reskin.

i assume this isnt a reskin but a swep?