Regarding GPX, I use Windows 7 64 bit. GPX still runs on it just fine.
But I had some issues getting it going at the start.

My solution was to use slic3r "post processing script" to define the path to GPX.exe then let Slic3r pass the parameters. This now works and I get x3g code produced alongside gcode.

Let me know if your still stuck and I can give you a hand.