Hardware unit to duplicate GD-ROMs, not used for development but useful for duplicating discs for testers.
You can see a video of the GD-X Duplicator by Adam Koralik here: (191) Keep Dreaming - Sega Dreamcast GD-X Duplicator HKT-01 - Adam Koralik - YouTube
The GD-Writer is used by developers to burn GD-R discs which are a writable version of the Dreamcast GD-ROMs.
You can see the GD-Writer in action in this video by Adam Koralik: (191) Keep Dreaming - Sega Dreamcast GD-Writer HKT-04 - Adam Koralik - YouTube
In order to use the GD-Writer hardware you would need to run a program called GD Workshop on your dreamcast DEV.BOX.
[^1] Keep Dreaming by Adam Koralik - https://youtu.be/kuff6PgoEuc