Leningrad survived because it was sieged by an overextended german army, who was not only facing the red army inside the city but also soviet troops at the east not very far away, the Finnish on the north also refused to go in. The truth is that the leningrad front was always a sideshow, the main battles of the war were at the center and the south of the USSR
Gaza is defended not by any army, but by a ragtag militia who are gonna face the blunt of the IDF forces, obviously way better equipped than the german army was a century earlier.
If you want to see what happens when a determined army really wants to stomp a ragtag militia with almost 0 resupply look at the warsaw uprising, 1 million poles killed or deported in 2 months even thought the german army was in a state of partial collapse, and still overextended, who knows if israel will reach that brutality, but there is a precedent when an army is angry enough.