The Granblue are a clan from the nation of Magallanica. These are a clan of pirates, most of them undead. They are focused on sending Units to the Drop Zone to call them from the drop zone. Their units typically have one of two types of effects; effects that send cards from the deck or hand to the drop zone like Zombie Guide, and effects that call units from the drop zone, like Monster Frank and Captain Nightmist.
In the anime, Gouki Daimonji uses a Granblue deck. Also, it should be noted that Rizak from Team Shinobi (Episode 71-72) used a Granblue deck as well.