Okay well, he is about 4 months old. I have him in a 75 gallon tank until I get this bid on a house, then I'll buy a 200 gallon plastic pond liner from lowes until he outgrows it. Five foot I will need a permit from DNR in Indiana. After, my dad does concrete, so I'll have him make an outdoor pond during the spring.
I live in Indiana as well. I researched keeping gators a couple years ago when I found one for sale at a petstore in MI for $60.
I'd skip the liner ponds, gators like to dig around in the mud at the bottom of ponds and I bet he'd tear it up quickly. A stock tank would be much better and probably cheaper as well (my 300 gallon stock tanks were less than $1 per gallon). Even better would be to just make the adult enclosure from the get-go and fence off the areas you don't want the gator getting into until he's bigger.
You're going to need an indoor, heated area that is split between land and water. The land section should be at least 10' x 30', the water section about as large. Seems big but not to a 6'-8' gator. Pool doesn't actually need to be all that deep, maybe 2'-3'. It would be best if you built an insulated pole barn so that you could have it set up to where you just open a door and the gator can get to his outside enclosure. Or move into a house with a walk-out basement so that you can set up much of the basement as the indoor portion and have it open to the outdoor enclosure.
In the end I decided that if I was going to sink that much money into a large, heated space I'd do it for an aldabra instead of a gator, but I wish you luck on your endeavor.