https://segaretro.org/Triforce#Media_BoardIn Namco's ROM board version the media board has the same network functionality as the Type 3 (and Type 1 if using a network enabled DIMM) but instead of using the GD-ROM drive it has an internal NAND based ROM cartridge and an SD card slot which was left unused. Most games will only either work with the Type 1 or Type 3 units and only the Namco ROM games can work with the ROM system.
But GP1 is rom cartridge type so, I can not find the the net booting.
If it's cartridge based, there is nothing you can do about it, other than replacing the media board for type 3 media board.Maybe you can't netboot on it?