They can hit 12" providing its a saum and not a true GT. A 50G if it is a 36x18x19" like i have on my list, is far too small for a fish that can hit 12".
Larger cichlids need a larger footprint, ie L x W. the minimum for a male GT should be a standard 55G but even that is pushing it because of the width.
From the way i am picking up your question it would seem you believe in the "fish only grows to the size of the tank" myth. Correct me if im wrong. If you do then you need to know that its false.
Providing you do proper maintenance and care then there is nothing stopping him hitting his potential other than genetics.