So, you've starting playing Garry's mod and joined a server with missing textures and errors everywhere! You have no idea how to fix them, most other guides show sketchy website links to download from.

The only legit and safe way to fix missing textures are other source engine games.
The most needed game is Counter-Strike:Source. I'll link a few others that you also might need.
You should wait until a sale where its very cheap.
https://store.steampowered.com/app/440/Team_Fortress_2/
https://store.steampowered.com/app/420/HalfLife_2_Episode_Two/
https://store.steampowered.com/app/240/CounterStrike_Source/
After that you go into Garry's Mod, and press the little game controller icon in the corner and tick off Counter-Strike:Source.

Finally go into the server or map you were on and, tada! Everything now has texture!
And
If you liked the guide consider donating steam points or sharing it with friends!