I have keep trying to solve the problem and finally found a solution for. I exported the Simulink file to a previous version (2013b) and surprisingly found that it worked (the new functional version attached). The only point to be considered is that the gear ratio should not be changed abruptly. It means similar to driving a sedan vehicle that we usually shift from 1 to 5 orderly, 1 to 2, 2 to 3, 3 to 4, and 4 to 5, the rate of gear ratio change has to be limited. It is why I have put a rate limit block in the simulation file.