Garry's Mod: Stable & Optimized

GMOD Prerequisites
(Required) Garry's Mod & Other Source Engine Games
Counter Strike Source & Half Life 2 are hard requirements. Most, if not all maps & addons you play on Garry's Mod, they use assets, textures, & models from either Half Life 2 or Counter Strike Source. If you don't have any of the games mentioned, you can go to those sites to download the content from the mentioned games, to fix 'missing textures/checkered textures/purple textures' you will get from not having the content. I will not link any sites that will provide the textures/content for you, you'll have to find them and download content from those sites at your own risk. I am not responsible for whatever happens to your PC.

Here is how to mount your games: (Go to 4:00)

YouTube: How To Mount Games in Garry's Mod
: 14,607
How to starts at - 4:00
GMOD Installation, Preparation, & Setup
(Recommended) Clean Installation of Garry's Mod
This process will reset your Garry's Mod configuration to default, delete your addons, and delete downloaded server files (You'll have to download the files/addons again when you load into servers).
Go to Garry's Mod Workshop & Unsubscribe to all addons.
Open Garry's Mod, press F10, type "menu_cleanupgmas", & press enter/or submit.
Exit Garry's Mod, right click the game, click "Properties", click the tab "Local files", click "Browse Local Files", lastly open "FactoryReset-GMod.bat" & press Y on your keyboard.
Uninstall Garry's Mod, & Delete the installation folder to completely remove Garry's Mod.
Reinstall Garry's Mod, & Verify Game Cache Integrity.

(Optional, for 64-bit Systems only) Running Garry's Mod in 64-bit
Running Garry's Mod in 64-bit mode will dramatically increase performance. Right click Garry's Mod, click "Properties", click the tab "Betas", & select "x86-64 - Chromium + 64-bit binaries".

(Recommended) Disable Fullscreen Optimizations for Garry's Mod
hl2.exe (\steamapps\common\GarrysMod):

gmod.exe (\steamapps\common\GarrysMod\bin) & (\steamapps\common\GarrysMod\bin\win64)

(Recommended) In-game configuration of Garry's Mod
Basic configuration of the options:
Keyboard: Click "Advanced" & Check "Enable Developer Console".
Mouse: Check "Raw Input" & Adjust Mouse Sensitivity to your preference.
Audio: Speaker Configuration should be either "Headphones" or "2 Speakers".
Video: Resolution & Aspect Ratio should be set to your native resolution, & use Full screen.

(Recommended) Advanced Video Settings Configuration
Under "Video" tab in options, click on the lower right "Advanced" to open this menu.
Model Detail - High (Some addons can be broken if this is set lower, however if you have a bad PC, then go lower.)
Texture Detail - High ("Very high" texture detail is highly not recommended for RP servers.
If your PC is bad, go Medium or Low.)
Shader Detail - High (Low if your PC is bad.)
Water Detail - Reflect All (If your PC is bad, go "Reflect World" or "Simple Reflections".)
Shadow Detail - Medium (If your PC is bad, go Low as it disables shadows.)
Color Correction - Enabled (If your PC is bad, go Disabled.)
Anti-Aliasing - 4x MSAA (8x MSAA is overkill, and if your PC is bad, go 2x or None.)
Filtering Mode - 16x (If your PC is bad, go lower.)
Wait for vertical sync - Off
Motion Blur - Off
Field of View - 90.00 or 100.00
Multicore Rendering - On
High Dynamic Range - Full (Some maps require HDR to be on.)

(Recommended) Enabling Multicore Rendering & Other Optimizations + Launch options
Enabling "Multicore Rendering" is an illusion in Advanced Video Settings, so we have to enable it through the console, or better yet a configuration file that will load the settings & save them to your config + saved in your user configuration cloud.
Right click the game, click "Properties", click the tab "Local files", click "Browse Local Files", then go to "garrysmod/cfg and right click autoexec.cfg & click open with Notepad or Notepad++. Copy & paste the following commands into "autoexec.cfg":

// Multi-core rendering settings
gmod_mcore_test 1
mat_queue_mode -1
cl_threaded_bone_setup 1
cl_threaded_client_leaf_system 1
r_threaded_client_shadow_manager 1
r_threaded_particles 1
r_threaded_renderables 1
r_queued_ropes 1
studio_queue_mode 1
mat_specular 0

fps_max 999 // Change "999" to the # of your refresh rate if you're on a laptop
menu_cleanupgmas // deletes unsubscribed addons
host_writeconfig // saves settings to config.cfg