If your garrys mod fails to start. When you try to open your garrys mod it might say hl2 is not responding. I have had this problem and I solved it. There are a lot of dumb things on the internet that say weird ways to fix it but all you you have to is uninstall are your steam stuff and all games and just redownload it this will give it a chance to get all new folders in the garrys mod folder so pritty much fixes it. BUT FIRST dont FORGET to take any iimportant add ons or anything that you really want and just drag it out of your garrys mod folder and put on desktop then obviously put them back in when your done downloading!!! Hope this helped!
. . . Wow.
I hope you know that about 99.9% of the community already knows this.
I didnt know and new to the site just had the problem and nothing anywhere said this so I am SORRY.