If you really plan on keeping that gator, for life, you need to build that tank as wide as possible. I really don't know anyone that has sucessfully kept a gator gar in captivity for long term that lived a healthy life, but you should count on at least 4 ft long gar. They do not turn well at all, so as wide as you can do it, the better. Long term, I would do a 12x6 footprint. Height is up to you