Israel never left Gaza. They surrounded it. They controlled the entire land border and didn't let ships sail or planes take off from Gaza. It's as if, after a prison riot, all of the guards just said fuck 'em and decamped to a perimeter around the prison with machine guns with orders to shoot to kill. Gaza is way too small and densely populated to be remotely self-sufficient.

Now, just to short-circuit the reply chain, you're going to say that Hamas would have just used the borders to move in weapons. Sure, that's true. In fact, they dug a bunch of tunnels under the Israeli controlled border crossings and smuggled in a bunch of weapons. Normal stuff like food and medicine though were constantly blocked or delayed.

By the way, I don't know how to solve this problem well at this point. Both sides have done enough evil shit to each other that they will want to kill each other for generations. Israel though, has never withdrawn from Gaza, and Israel has never agreed to abide by the terms of UN Resolution 242 that obligates them to withdraw from the 1967 occupied territories. What I expect to happen based on the ground reality and statements from members of the Israeli government is that the Palestinians in Gaza will either die or have their lives be made so intolerable that they leave, and then Israel will annex the land, just as they have in Jerusalem and many areas of the West Bank.