In some Japanese dictionaries, you'll find that グランド （grand) and グラン (gran) are listed together to mean "large," "big" or "important". Jib71 touches on it above, for example, Grand Prix in Japanese is written グランプリ （Gran Pree) with a silent "d" just as the original French is pronounced, so you can think of "gran" and "grand" as meaning the same. So "Gran Class" on the Shinkansen is an "important" (VIP) class.