I have a few thoughts. Where are you located? Green house should help with temp but gator gars do better in warmer water, and if your going 4 feet deep your going to have to fight 50F ground temps so heating will be a pain. I imagine it would be difficult to keep temps up in winter, but I know almost nothing about green houses.
I feel as if though 5' width won't be enough for a gator gar long term, as they can get quite a bit larger than 5'. I'm not sure how long you plan on keeping them or if you plan on harvesting them as well. Realistically if it's only 2900 gallons I wouldn't put a single gar in long term.
If your going planted only you could probably save a bundle on ceramic and other media.