There should be links to a compiled gunz client that you can debug and decompile, so I'm just going to make a tutorial on how to start GunZ in debug mode.
Step 1: Create a shortcut of the launcher skip (GunZ.exe not GunZLauncher.exe)
https://i.imgur.com/lt4smoS.png
Step 2: Right Click on the shortcut and go to "Properties" Now find Target:
https://i.imgur.com/H41N1Kk.png
https://i.imgur.com/sxolNkT.png
Step 2.5: Find target and after the existing text write /game "Mansion" debug
https://i.imgur.com/WeLMJVo.png
What "Mansion" is, is the map name, so you could, for example write "Skillmap1" instead (if you have it in your maps folder) and it will load up the map "Skillmap1"