The territory will be overseen by the board of peace, which is chaired by Trump. The details of the plan show that most of the decisions aren't taken by the board but by its chair. Trump will be de facto leader of Gaza. Technically you're right, it's not going to be an American colony. It's a Trump one, and that's even worse.
For the foreseeable future, Gaza would be a Trump colony, with zero democracy, and with Palestinians having zero say.
The plan goes into detail about investment, partnerships, etc to ensure a good return on the building projects. No coincidence that Jared Kushner is a key individual in this plan. Jared who is talked repeatedly about the 'real estate opportunities', and happens to an a company that would make billions off it.
Do you really think after this America is going to leave? There's nothing to stop America plonking a huge military base there. America previously expressed a wish to annexe Gaza (along with Canada and Greenland!), doesn't concern you that this plan comes suspiciously close? In reality, it would be a colony, but one with absolutely no accountability. Trump could do whatever he likes.
Also, the proposed 'full Israeli withdrawal' which Netanyahu agreed to, he has retreated from this morning, saying the IDF would stay in most of the territory. So actually not a withdrawal at all.