You can use the Planet CNC MK2 controller with an SD card addon to run the gcode without needing it connected to a computer.
Alternatively, as it connects with USB you can use it with the Mac running Windows virtualised as well. Comes with it's own software which is quite good but not quite got the versatility of Mach or LinuxCNC just yet.

Planet CNC
I've just picked up two of those from Zapp Automation. A bit overkill for what I need personally but I can always make use of them as things progress.

Obviously it'd mean buying a new controller though.