A LIST OF KNOWN GARRY'S MOD ERRORS/PROBLEMS + FIXES
Garry's Mod is known for being unstable, with alot of problens and bugs.
In this thread I will post some of them and the fixes, if your problem is not listed here reply, and I will get back to you as fast as possible.
GARRY'S MOD WONT LAUNCH WIPHOUT ANY ERRORS
This is mostly because its still updating, launch it, wait and it should start after a while, otherwise try to go to your Garry's Mod folder, left click on the .EXE and do properties, from there change the OS to your current OS version. (Windows only.)
GARRY'S MOD IS LAGGING
There could be alot of things causing this, frist check your ping, if its not your ping try another server, if you keep having the problem it might be that all the servers are hosted in a location far away from your location, causing lag. (ex: People from Europe could have lag from servers from USA)
LONG SERVER LOADING TIMES
-Do you got a bad ping like listed above, or is the server far away from your location? Try joining a other server or gamemode and look if that fixes the problem, this could aslo be because of alot of custom downloads from the server your joining.
If you join a server and you see "missing map ****** disconnecting'' that means the server has a map that you have not downloaded, you can fix this by searching the map name in the steam workshop, download it there and relaunch Garry's Mod.
If you are missing textures, make sure all Garry's Mod textured games are downloaded and launched. (Counter Strike: Source and Half Life 2.) If you dont got those two games, you can always download its textures illegaly trough sites like pirate bay.
SERVER LIST DOES NOT LOAD
Facepunch is aware of this problem and clamed they "fixed it" but some people still have it. Mostly its better if you relaunch Garry's Mod till it works, if this does not work you could try to reinstall it.
Completely exit Steam, then navigate to your Steam installation folder and delete the 'appcache' folder. Restart your computer and relaunch the game.
HL2.EXE HAS STOPPED WORKING
Go to your Garry's Mod folder, left click the launcher and change its mode to the newest OS. (This could not work if you dont got Windows 8 or 8.1)
IF ANY OF THESE PROBLEMS DONT WORK, TRY TO VALIDATE GARRY'S MOD FILES.
Like stated above, if you need more help just reply!