Software is corrupt that worked only months ago?
Apple's OSX Mavericks 10.9.5 update changed some app signing rules.  I'm not sure if that's exactly what's happening with JD-GUI, but it seems plausible.  

To bypass the Gatekeeper check that's popping up the error, launch the app directly from a Terminal prompt:

$ /Applications/JD-GUI.app/Contents/MacOS/jd-gui &amp;