Bad product and after service

Bought a brand new Opel Adam Jam. Had an instant problem with the gear box, a constant grind in and out of 4th gear. Took the concern through all services offered at Opel, e.g. repairs, management etc, They replaced the gear box and it still continues. The same issue we have has been reported on other vehicles as well and yet still no concern about the problem. Took it up with GM themselves in PE and were told that it will not affect the performance of the vehicle, case closed! This is such bad after service for a brand new car with a serious gear problem.