Israel can’t annex Gaza.

Neither the people of Gaza, nor the world would allow it.

The UN basically never takes affirmative military action, but even if they don’t make an exception for Israel (the way they do in condemning a nation’s behavior), they’d be happy to coordinate a global embargo and to fund the coalition of the dozens of states that have spent the past half century waiting for the means and opportunity to wipe Israel off the map.

Also, there are over 2 million people in Gaza, and a pretty big proportion of them would rather kill an Israeli than live another day themselves.