The launcher is supposed to close after it tells the kernel to start the client's exe.
It's possible that the launcher is trying to open the executable, but by the wrong name.
Here's what you can try:
If your client's exe is named "Gunz.exe", change it to "theduel.exe", or if your client's exe is named "theduel.exe", try naming it "Gunz.exe".
- OR -
Open a hex editor like Hex Workshop,
search for a text string that matches up with "Gunz.exe" (without quotes) (If you couldn't find anything that matches with "Gunz.exe", try "theduel.exe"),
then change the string you found to your client's executable name.