The air taxi pilot related he was flying a company airplane on a personal flight in a mountain pass when the weather began to deteriorate. This was the pilot's third flight of the day in the same pass. He said he turned the airplane around in an attempt to exit the pass, but the clouds had lowered and he could no longer see the exit to the pass. He elected to make a precautionary landing on the tundra in lieu of attempting to continue the flight through the pass. The main landing gear separated from the airplane during the landing.