well the gator will be good for about 5 months tops i would think
not too sure on there growth rate tho how ever ive heard that it can be up to 2-3" a month
that gator will reach 2-3 feet in his first year. they are much more aggressive than the floridas and would think nothing of killing it when big enough. do you have a bigger tank or a pond ready. that 120 will soon be a mud puddle for that fish. the florida will be ok in it for a while, but will also outgrow it.
not long. like i said...he should be around 2-3 feet in about a year. your 240 is probably 2' wide? i think that would give you about a year. they are a fish that has very high demands.