Here's the UN's definition of genocide...

"any of the following acts committed with intent to destroy, in whole or in part, a national, ethnic, racial or religious group, as such: killing members of the group; causing serious bodily or mental harm to members of the group; deliberately inflicting on the group conditions of life, calculated to bring about its physical destruction in whole or in part; imposing measures intended to prevent births within the group; [and] forcibly transferring children of the group to another group"

Israel's refusal to allow Gazans to leave the strip prior to bombing it indiscriminately (using weapons designed to kill large numbers of people indiscriminately), and its refusal to allow the people in Gaza to have access to food, water, fuel for heat, medical aid, and the materials needed to provide themselves with shelter (after their homes were destroyed by Israel) in the middle of winter, all support the use of the word genocide. I wouldn't say that Israel has already committed genocide. But I think it is probably accurate to say that Israel is in the process of committing a gradual kind of genocide, and that it will not stop until there are no Palestinians left in Gaza.