You reuploaded the mod by a wrong way, mod was firstly intended to be installed by using "custom" folder, but hl2 workshop loads mods by working with .vpk files that contain normal files, and the problem is that you reuploaded mod folder that contain .vpk files, so workshop packed .vpk files into .vpk file and seems like it can't work with .vpks that contain other .vpks, so it's loading absolutely nothing. To fix it you need to unpack .vpks from mod's folder to other folder, it should be like:
(mod's folder to reupload it on steam)
materials
(mod's mats)
models
(mod's mdls)

this doesnt work

Is there a standalone mod for just the turrets?

This doesn't change the models, am I doing something wrong?