Rainer Schaller -- who owns Gold's Gym, among other brands -- is missing alongside his family after their plane disappeared overnight ... with officials all but confirming they're dead.
A private plane carrying Schaller, his partner Christiane Schikorsky, their children and a reported fitness trainer are listed as the five passengers who have gone unaccounted for since their aircraft vanished off the Costa Rican while traveling from Mexico Friday.
Reports say the plane lost all communication with a control tower while it was flying near Barra del Parismina in the Caribbean Sea -- and now, the Costa Rican Coast Guard is saying they've found several pieces from what they believe is the wreckage, including at least 2 bodies.