I decided to build my own USB loader for GMC-4. Following the advice of Serg Vakulenko, the following board was purchased:

This is a USB Bit Whacker, a development board based on PIC18. Its default firmware allows to control I/O pins via simple commands over a virtual serial port.

Schematic.

A prototype.

I was lazy to etch the board, so soldered manually. Looks unsightly but works.

In action.

After all, GMC-4 is THE thing!

P.S. The sources are available on Github – gmc4-loader. I suspect that in Python or Ruby it might be much nicer, but it was a nice exercise programming serial ports in UNIX.