The Gaza Strip is about 140 square miles, roughly twice the size of Washington, D.C, and is home to about 2 million people. By contrast, Manhattan Island, NYC, is about 22.5 square miles and is home to about 1.6 million people.
Putting the entrance and exit issues aside, I'm not sure how Gaza, then, is described as "some of the densest concentration of people in the world". I'm well aware that you didn't coin the phrase but still not sure what it means, exactly and how it's gotten so much traction.