From what I understood the following is readily available:
- Schematic of the GMX (last page of the manual -
http://zxspectrum.it.omegahg.com/doc/gmx.pdf)
- Contents of the FlashROM (
http://zxspectrum.it.omegahg.com/rom/gmx.rom)
- Firmware of the CPLD (Altera7064SLC_for_GMX.pof - JTAG disabled in file, needs special "in socket"-programmer)
Recreating a layout from the schematic should be no big deal. The current challenge would be to find (someone with) a suitable programmer and verify that the .pof file works in a GMX board that's known to work.