You need to have the Extended Spawnmenu for the folder(s) to appear in the spawnmenu.

Open the RAR or ZIP file, then drag and drop (or extract or copy) the folder into the "addons" folder. It's located in:
C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod

So the full path would be this:
C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\addons

If you have the GMA files for these addons, you can place them in the "workshop" folder, which is located in:
C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\cache\workshop

You could also try placing them in the "addons" folder, but it has to be inside of a folder. So create a new folder, then drop the GMA file, there. For example, "Splatoon - Uniform Textures.gma" would go inside the "Splatoon - Uniform Textures" folder.

They should appear under "Browse Materials", though I haven't tried this, myself. I've only done this with some models, so I'm not sure if it'll work or not.

Add more realism to your work!

This pack includes textures for shirts, sweaters, aprons, hats, and more. Most of them are accurate to their real-life counterparts (though I’ve taken creative liberties with some of them). Now you no longer have to settle with the stock textures!



Includes textures for:
-Apple
-Google
-GameStop
-Best Buy
-Starbucks
-McDonald's
-UPS
-Paintable shoes, clothing, aprons and hats
-Burger King
-Dairy Queen
-Pizza Hut
-7-Eleven
-KFC
-Subway
-Tesla
-Lifeguards
-Police, fire, and medical
-Sega

And more!

The Source Filmmaker version is available, here:
https://mega.nz/folder/oI532TbL#DoA5RXp9CTDBzpcMS_L7GQ

Link to the SubMaterial tool: https://gtm.steamproxy.vip/workshop/filedetails/?id=405793043
You also need the Splatoon 2 : Leggings and Shorts addon.

To use this, you need both the SubMaterial tool and the Extended Spawn Menu. You can find the textures in the Materials/Browse Materials tab in the spawn menu. Scroll down until you see “Splatoon - Uniform Textures”.

Expand the “models” folder. Find the texture you want to use and right-click it. Copy it to the clipboard, then go to the Tools menu. Select the SubMaterial tool. Paste the texture path into the search bar (make sure it appears exactly as it should or it won’t work) and press Enter. Then use the tool to apply it to the clothing or headgear.

Credits:
VortexBlue – Creator of the textures


MEGA: https://mega.nz/folder/FZYnQSLI#ROFABQDBJuU8918OQ9yO4w
MEGA (SFM version): https://mega.nz/folder/oI532TbL#DoA5RXp9CTDBzpcMS_L7GQ

VortexBlue (direct download): https://vortexbluecreations.com/uploads/s/u/g/r/ugrcqsw5bq5q/file/ZKBNkQSY.rar

Discord: https://discord.com/channels/1232783710492037183/1232809452957208577

Meggy's outfit from Glitch Productions' animated web series Sunset Paradise is now available for Garry's Mod!

This pack includes textures for her shirt and shorts.

Requires:
- SubMaterial Tool
- Splatoon 2 - Clothing
- Splatoon 2 : Leggings and Shorts
- Splatoon 2/Spoon - Octolings [PM/RAG/VOX]
- Extended Spawnmenu

MEGA: https://mega.nz/folder/FZYnQSLI#ROFABQDBJuU8918OQ9yO4w

VortexBlue (direct download): https://vortexbluecreations.com/uploads/s/u/g/r/ugrcqsw5bq5q/file/kFe3XAV0.rar

Discord: https://discord.com/channels/1232783710492037183/1232809452957208577

Includes:
-2 hairstyles -- Sunset Paradise and regular hairstyle
-Meggy's outfit

How to use:
Select the Weld tool, and press R to remove the weld constraints.

Requires (SMG4) Inkling Meggy - Sunset Paradise outfit[vortexbluecreations.com], and:
- SubMaterial Tool
- Enhanced Inklings
- Splatoon 2 Cosmetic Pack
- Splatoon 2 - Shoes
- Splatoon 2 - Clothing
- Splatoon 2 - Headgear
- Splatoon Socks and Phone pack
- Splatoon 2 : Leggings and Shorts
- Splatoon 2 - Octolings [PM/RAG/VOX]

MEGA: https://mega.nz/folder/FZYnQSLI#ROFABQDBJuU8918OQ9yO4w

VortexBlue (direct download): https://vortexbluecreations.com/uploads/s/u/g/r/ugrcqsw5bq5q/file/sSDgPNd3.zip

Discord: https://discord.com/channels/1232783710492037183/1232809452957208577