I am looking for a cross platform Synergy GUI.
I don't know of any one GUI that works on OSX/Linux/Windows. However, the raw config file is easy to read and understand. It is well documented, and only takes a moment to set up.
You basically need to call out screens (computers you are controling), links (computer1 is up/down/left/right of computer2), and aliases (host names of the computers).
That being said, http://code.google.com/p/quicksynergy/ works on OSX and Linux, and I believe the windows version comes with a GUI.