This guide is outdated, but appears to function just fine, take that as a warning.

If you care about performance, Garry's Mod is almost entirely CPU dependant even with all these graphical enchanements.
It doesn't matter how top of the line your GPU is, you will still have low fps if your CPU is lacking, for example a stock FX 6300.

HD textures Part 1:
http://www.moddb.com/mods/hd-world-by-clipodel
Download then extract the .7z file using 7-Zip which you can find at http://www.7-zip.org and open the materials folder then goto your GMOD materials folder and drag everything in the HD World's Materials folder to GMOD (Overwrite conflicting files!)

HD textures Part 2:
https://mega.nz/#!qIdAxKoK!PI0pzGobyBG-EBNdG6yEbjxqaoXKxSVXMomgjhml2Z8

To make GMOD DirectX 9.8 (Helps with quality)
1. Right click Garry's Mod and click Properties
2. Click Set Launch Options
3. Type in -dxlevel 98
4. Now run GMOD with these settings then repeat the following steps but remove the -dxlevel flag from the launch options
5. Now it will stay running on DirectX 9.8

Now change all of these in the post processing effects tab


Shaders Tab:

Enable Color Modification

Brightness from -0.15 to -0.10
Color Multiplier from 0.500000 to 1.00 or 1.20 if you want more colour!

Enable Bloom

Enable Toytown

Height: 0.40

Enable Sun Rays

Multiplier: 1.90
Darken Amount: 0.97
Sun Size: 0.15

[RECOMMENDED] Enable Sharpen

Distance: 0.99
Contrast: 0.45 or 0.40 if its too much!

Effects Tab:

(a bit laggy,and its mostly for screenshots): Enable Depth of field and put its settings as:

Space between nodes: 3024.00
Intial node distance: 3024.00

Enable Accumulation Motion Blur And Put its settings as:

Add Transpency: 0.75
Draw Transpency 0.99
Frame Capture Delay 0.00