There are a lot of variables involved in the growth rates of fish. Clean water is probably the most important aspect of allowing a fish to grow to it maximum potential, 50% weekly water changes, a stable/suitable environment and quality food are keys to getting the most out of a particular species. Most hobbyists over feed to obtain quick growth, this is not however good for the fish in the long run that could lead to skeletal deformities and damage to internal organs such as the liver.
GTs are omnivorous however require more vegetable matter in their diet than most other omnivores. Foods high in animal protein are more difficult (take longer) to digest and tend to cause bacteria to build up in GTs digestive tracts leading to a lot of the problems people encounter keeping these fish.
The sex and genetics of the fish also plays a role in growth rates and generally females dont grow as fast or as big as males. Dominance is also a factor, I have had situations when keeping more than one male GT together and the dominant fish grew at a steady rate with nice color and the sub-dominant male stayed smaller and didnt color up like the Boss did.
Since you didnt give specifics as to your tank size, stocking, water conditions, decor ect. I gave a broad answer. In short keep the water clean, provide a comfortable/stress free environment and feed a diet high in vegetable matter and it will grow steadily and healthy at its own pace.