In August, the IPC found about 514,000 Gazans are Phase 5 (famine / humanitarian catastrophe) [1][2]. It projected by September that was around 641,000. So the threshold you're looking for is a crude death rate (CDR) between 100 and 120 per day.
CRD "needs to be directly attributable to outright starvation or to the interaction of food consumption deficits and disease" and does not include trauma [3]. So it will be more than just confirmed deaths from malnutrition or starvation (which is, in practice, impossible to procure for anywhere on even the brink of famine).
[1] https://www.ipcinfo.org/ipc-country-analysis/details-map/en/...
[2] https://en.wikipedia.org/wiki/Integrated_Food_Security_Phase...
[3] https://www.ipcinfo.org/fileadmin/user_upload/ipcinfo/docs/I...