Casio G-SHOCK GR-B300-1A4DR GRAVITYMASTER WatchMASTER OF G AIR GRAVITYMASTER GR B300 1A4 The skys the limit! Introducing the GRAVITYMASTER GR B300, a piece of aviation inspired professional gear designed to work even in the harshest environments, from the G SHOCK MASTER of G line. The GR B300 features a Carbon Core Guard structure, which delivers the shock resistance, rigidity, and corrosion resistance of carbon material used in aircraft fuselages. The carbon fiber reinforced resin case and the