Good luck with that but you can prove or disprove growth rate depending on food availability, type of food as well as environment. There are many different factors that go into the growth of a fish.
On average a properly housed Gator gar (on prepared foods) will grow 5mm a day after 10days after hatch. This will roughly work out to a 30cm fish in four months.
This info BTW can be obtained easily with about 30 seconds of your time and a wonderful thing called google:
Studies on the physiology of Atractosteus spatula larval development and its applications to early weaning onto artificial diets ...
This is not "my opinion" it is a known fact these fish have one of the fastest growth rates of freshwater fishes. There is plenty of info (not anecdotal) out there that confirms this.
That aside I personally could care less if you got the fish and my question to you was more related to your original question. If it is a gator then feeding will be different than if it is a Lepisosteus and you need to take a different route to growing them out. Atractosteus gars are very different from Lepisosteus in general care, this is why proper ID of the fish is really needed should you actually care to be successful with them.
For the record my little gator (of which you can find a thread here on) is in a 55 gallon right now...(I suggest this for small gars) He is right on track with growing 5mm a day on prepared foods and has not slowed in the least..