To begin, you will need to have "Crowbar" app which you can download from original page .
Once you downloaded it you will see this when you open it (you will need 7z to open it):
as you can see in text you need press refresh button. So go ahead and press it




Now you need to follow this simple steps:
And choose two tags about your map.

Now you need to create few folders.
Create first folder and name it "maps"
then you can put your map (.bsp) inside.
Create second folder and name it as you want, i usualy name it as my map name:
put it inside folder 2

first we need open our map in crowbar
then we can add image for map
After all steps you can finally press "Publish" button!
you can check your map in workshop by pressing this button:

As you can see your map doesnt have preview inside Gmod
to create thumbnail you need create "thumb" folder inside ur maps folder











create image for thumbnail that MUST be 512x512 and .png its important!
put your image inside "thumb" folder and name it same as your map name











now when you ready we can update map with thumbnail!

To update map (for bsp change or thumbnail/ any change)
you need upen your map folder again in crowbar and press "Update" button.
you can also write what changes you did in change log:

Some times when you did something not right you can get errors and your map wont be published.
Here few errors i know how to fix:
------
Processing content for upload.
Creating GMA file via "N:\steam\steamapps\common\GarrysMod\bin\win64\gmad.exe".
Garry's Mod Addon Creator 1.1
Looking in folder "C:/Users/25vla/Desktop/mg_parallax_v10/maps/"
Removing ignored files...
Verifying file list...
mg_parallax_v10.bsp
[Not allowed by whitelist]
thumb/mg_parallax_v10.jpg
[Not allowed by whitelist]
The following files did not pass verification:
mg_parallax_v10.bsp - Not allowed by whitelist
thumb/mg_parallax_v10.jpg - Not allowed by whitelist
File list verification failed
ERROR: Processing content failed. Review log messages above for reason.
------
This error happened because in map derection you put /maps
C:/Users/25vla/Desktop/mg_parallax_v10/maps

to fix it just remove it(/maps).
it should look like this:
"C:/Users/ur_user_name/Desktop/mg_parallax_v10"

------
Processing content for upload.
Creating GMA file via "N:\steam\steamapps\common\GarrysMod\bin\win64\gmad.exe".
Garry's Mod Addon Creator 1.1
Looking in folder "C:/Users/25vla/Desktop/mg_parallax_v10/"
Removing ignored files...
Verifying file list...
maps/mg_parallax_v10.bsp
maps/thumb/mg_parallax_v10.bsp.jpg
[Not allowed by whitelist]
The following files did not pass verification:
maps/thumb/mg_parallax_v10.bsp.jpg - Not allowed by whitelist
File list verification failed
ERROR: Processing content failed. Review log messages above for reason.
------
This error happened because you used .jpg image for your thumbnail.











to fix it just make it .png