My brother and I both own a copy of Gmod. However, only I own Css. Can I use the family share function to let him download counter strike and use the textured for Gmod?
If you both play the game on the same computer the textures will be installed and copied over anyway. However if not you could go to this website go to the garrycontent page and scroll down to the part where it says Source Textures or something like that and add it to your
C:\Program Files (x86)\Steam\Steamapps\Common\Garry's Mod\addons
This will install the textures for you no matter what :D
This will probably not help you in any scenario, but there are multiple CSS Textures already in the Steam Workshop of Garry's Mod: https://steamcommunity.com/workshop/browse/?appid=4000
They are freely downloadable.
I didn't find much if family sharing would fix the problem, but I would give it a try since you own it anyway.
You don't need to own CSS to get the textures for Garry's Mod! Most people I would say don't own CSS :p It's also legal so no Issues there.
Link: GMOD Textures
Literally only takes a couple minutes. That page has a video tutorial and the download links.