Pink textures in gmod

Depending on what map you're playing, it could be one of a few games but your most likely culprits are:

Counter-Strike: Source
Half-Life 2
Half-Life 2: Epi 1 & Epi 2

Its worth noting that you can get the Half-Life 2 textures for cheaper by buying Half-Life 2: Deathmatch, then avoiding the full charge

You can find downloads as stand-alone addons for Epi 1 & Epi 2's materials and models. They're still up on garrysmod.org I believe

Make sure the server you're playing on; if it has a workshop collection that you're subscribed to it