Missing folders

I have a big problem with garry’s mod.
I’m missing the materials folder, so the game won’t work.
And it said that a background is missing, everything in the
folder is missing too.
Please help XC

Um, if you bought it from Steam(which you should of) go to your game library and right click GMod, then click [Properties] click the [Local Files] tab and press [Vertify Integrity of Tool Cache] and let that go through. If it is still missing the folder, unistall GMod via windows uninstall program and then go back to steam and right click GMod and click [Install Game]. Hope this helps!