Slic3r checks what you type in that box to make sure it is an executable on Windows. It has to be a full path to a thing that can be run and no parameters, so your box has to have a full path to where GpxUi puts a version independent link to gpx.exe:
C:/users/yourusernamehere/appdata/gpxui/gpx.exe.lnk
Or something like that. I'm away from my keyboard at the moment.
If you open up explorer and look at c:/users, you should be able to navigate down to find it.