Ah it's getting juicy!
GDR coming from Japan with System Disc does obviously point to a Japanese non released game.
If it is Japanese game originally developed over there then it rules out numerous western titles but there are still plenty of possibilities.
PCWizard stated that it's "HUGE news" so it's unlikely to be an unreleased hentai game or the like.
It could be a game previously released but a later build like Geist Force or HOMM III for example but I think it'll be a new release. Gorka Morka perhaps? it'd be amazing if it was something like Gun Beat or Chakan
As you can tell I am very excited about this so I really hope it isn't a Hentai game!
Dee Dee Planet is still my first thought as it is confirmed to exist and I think a collector does own a copy somewhere in the world.
If I could wish for one game I would love it to be my personal grail Gun Valkyrie I was so gutted when it was canned