If you don't own the following games, you can download content-packs below:
Extract inside your garrysmod/garrysmod folder
Counter-Strike: Source - https://ggs.sx/downloads/css_content.rar
Half Life 2: https://ggs.sx/downloads/hl2_content.rar
Make sure you're subscribed to all the addons in this collection (Click Subscribe to all)
Make sure you have room on your Hard Drive.
If you're experiencing endless errors, can't see HUD etc. I recommend reinstalling Garry's Mod by deleting/renaming garrysmod to something else.