It works as a Mouse/Keyboard when set in Mouse mode the cursor on the screen moves but doesn't seem to register in RetroArch.
I finally got it to at least register the buttons and triggers on the gun in RetroArch and was able to map controls. It wasn't Controller Input I needed to go into, but RetroPad Binds. That at least let me set it to the Lightgun, registers Trigger button, Reload button, set the Start and Select. However, when in the game, it still doesn't respond. It seems my Joystick is buttons are still mapped or take priority. I did try duplicating my RetroArch directory, creating a new Emulator called RetroArch Lightgun, so I could set the primary controller as the default, but so far, I'm still having an issue.