Gaza City *is* the most crowded place in the world. According to studies by many independed organisations. (UN organisations, and private non-profits).
The Gaza Strip on the other hand isn't the most crowded place, because around 40% is agricultural area.
Do remember that the diffrence between Gaza and HongKong is in standard of living, and not just population.

If you compare the urban parts of Gaza with Gush-Dan, New-York or even Paris, wou will find that Gaza is much less crowded. Except of some new parts of Gaza, most of the houses are 1-2 floors only.
You can look at all these if you visit Mari's observatio point, for example, near Sderot