How can you drive a car for 6 months and then only discover a problem with the gearbox?

I have never heard of a roadworthy test for the reliability of a gearbox!

How was the car licenced to the owner if it did not go through roadworthy?

Nah, I think you're out of luck on this one. Cars break just like all other things and anyway R 1,250 to repair a gearbox is in real terms not a lot of money - I have to spend R4,600 just to replace a clutch on a 9 year old Opel Corsa and we just spent R 700 on a Uno 1100 clutch kit.