Why isn t the Gaza Strip not listed in the UN for military occupations ?

Gaza is still in fact militarily occupied. For an area to be considered occupied the occupying state must exercise “effective control” over it. Unlike in the past, the Israeli siege, surveillance and monitoring technology allow for effective control of Gaza through controlling select key positions without the necessity of a full occupation force inside the area.
This position is echoed by the United Nations, Amnesty International, the International Red Cross and countless other international organizations specialized in human rights and international humanitarian law.
Further reading:
Was the Israeli disengagement from Gaza an act of peace?