GMod 14.04.19 missing textures (HELP)

I am new to GMod but have wanted for long time. I just purchased GMod off Steam from the sale today, and am having missing textures when I play. (Purple and Black checkers), or no textures and warping happens.

According to many search results on Google to do with the newest builds of Gmod it states that GMod doesn’t need Source games to be purchased as well for textures to load properly?..

Please help me fix this, I’m no expert so any help would be appreciated to help me solve this!

Most multiplayer servers require CS:S content as mentioned on the store page.