OK, some progress - I've connected and can control the printer via Repetier Host, yay!
The gMax's Arduino doesn't show up at all in PrintRun/Pronterface:
The Arduino IDE and Simplify3D Creator can see the port(s), but neither can connect. The Arduino IDE doesn't go up to 250K baud; Creator throws up a message that the Mac's native drivers can't handle 250K and it fails when it tries to communicate. That's a shame because I'd really like to use Creator.