This guide will explain to you how to extract downloaded Workshop items in order to modify them.

GmadExtractor
BSPsource (optional) *Requires Java*
MilkShape 3D (optional)
VTFEdit (optional)
VIDE (optional)

Downloaded Workshop items are stored here in your Garry's Mod addon directory:
To extract the gma. files you need Gmad Extractor. That's how it works.

Download the program. Either google it or use my download link:
https://www.media fire.com/file/p2vap0ve9cqtngq/GmadExtractor.zip
Remove the spaces between "media" and "fire", to open the link.
Virus Total (in case you wondering)[www.virustotal.com]

Now to the extraction part:

To modify maps you need BSPsource and Hammer.
Download BSPsource from GitHub [github.com]

Maps are stored as .bsp files, which the game can read, but your Editor can't. To make a map file modifyable, you have to convert the .bsp file to a .vmf file, which your Editor can read.

Here's how you do that.

1. Run bspsrc.jar.
2. Click Add.
3. Choose any .bsp file
4. Click Decompile.
5. You're done.

Some maps might be protected of decompiling. There is no way around, as far as I know.
Here is a guide by TopHattWaffle, explaining decompile protection.
https://www.tophattwaffle.com/protecting-a-map-from-decompiling/

Once decompiled, you want to edit your map with the Hammer Editor.
To get Hammer, you need to have any Source Game installed. Hammer will be automaticly installed when you download Half-Life: 2 or similar Source 2006-2011 titles. The program is located here:
C:\Program Files (x86)\Steam\steamapps\common\Half-Life 2\bin\hammer.exe
For other games, it's basicly the same. Here's an example for Counter-Strike: Source:
C:\Program Files (x86)\Steam\steamapps\common\Counter-Strike Source\bin\hammer.exe


If the map contains custom textures and models you have to extract them out of the .bsp file.
Use VIDE (http://www.riintouge.com/VIDE/) to extract all the resources you need. Then simply unpack the zip-file you get into your materials/models folder.


That's pretty much it. Open the .vmt file and edit the map.
Please consult other guides for mapping advice.

In Source, models are stored as .mdl files. To edit them, use MilkShape 3D.
View other guides for in-depth explainations, please.

Materials consist of a .vmt file and a .vtf file. The .vtf file contains the texture itself, while the .vmt file is a text document telling the game how to interpret the material.
It usually looks like this:

To edit .vtf files you need VTFEdit, which can convert these files into .jpg, .png or whatever. VTFEdit is also able to convert regular .jpg files to .vtf files.

Converting a texture is only half the work, because you need a corresponding .vmt file. It can also be created by VTFEdit.
This guide by WorldOfLevelDesign describes the whole process with pictures and short explainations.
http://www.worldofleveldesign.com/categories/wold-members-tutorials/keith_garry/custom-textures-hammer-keithgarry.php
If you're more into videos, there are a few good ones by 3kliksphilip and TopHattWaffle.
3kliksphilip's Video
TopHattWaffles's Video

If you plan packing and releasing your edited addon on the Workshop, you'll have to use the Gmod Addon Creator V1.16 (GAC)

Originally posted by Scarention @ gmod.facepunch.com:

GAC is a wrapper for both gmad.exe and gmpublish.exe and makes creating, publishing, and updating addons on the Steam Workshop much easier. I've also included gacxlib.dll in case you'd like to use it in your own program. (of course, you'll have to try to figure out how to use my mickey-moused functions) KEEP ALL FILES IN ONE DIRECTORY OR IT WILL NOT WORK.

Requirements:
-.Net Framework 4
-Garry's Mod

Features:
- Compile GMA files and upload / update directly after compiled
- Compile GMA for uploading later
- Upload GMA to workshop
- Update existing workshop addon with a GMA
- Create an addon with literally no experience whatsoever
- Create JSON file for addon
- Accepts PNG, JPG, and GIF Images (512x512)
- Does all the batch files for you (no messing around with CMD)
- Makes uploading an addon a hellofalot easier
- User Interface no longer freezes at any time
- Error report generated to desktop in a .txt file
- Extract GMA Files to specified folders

If you have any detailed questions concerning GAC please consult the offical Facepunch forums as they can help you more than I can.

I hope that the guide was somewhat helpful. Please ask me if anything was unclear.

Guide written by 「 Fluffy 」 @ society.gg.