I decided to build my own USB loader for GMC-4. Following the advice of Serg Vakulenko, the following board was purchased:
I was lazy to etch the board, so soldered manually. Looks unsightly but works.
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.