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It's been ages since I've looked into this, so my memory is probably bad on the details, but my understanding is that GD-ROM is very close to standard CDs in most respects, and can even be read in some PC optical drives with the right procedure (which I think involved either hacked firmware or hot-swapping between a specially crafted CD-R and the GD-ROM, to trick the drive into reading the high-density area). The main CD interface that Sega used is an off-the-shelf Oak Technology chip, but not driven by stock firmware. The high-density area was in a second session that was not linked in the first session, but rather was at a fixed offset that the firmware knew to look for. The high-density area basically worked by spinning the disc at half the speed that would normally be used for the interface chip. Since the disc was recorded the same way, the signal coming off of it looks exactly like a regular CD signal (maybe with a bit worse SNR).