DreamCast article updated with info about GD-CD's.
"..GD-ROMS use normal CD's and normal Cdr.'s to write them. The extra storage space on a GD-ROM is created by using the space usually reserved on a CD for sector headers and other such info. Also, the 2nd session uses CAV which makes this a whole lot easier. The crucial part is the SCSI controller, which is able to send the correct commands to the disk to read/write this information. This is how the dev-kits work. The official Dreamcast burners are just about any model of Yamaha burner.."