- MSF provide health services and are around 80% efficient (20% of your donation goes to overhead). I'm not sure if they make it easy to earmark a donation to Gaza.
- UNICEF does more infrastructure projects, but have around 30% overhead.
If you really want to maximize your impact, check if your employer or professional association have donation matching for various large charities.
There are obviously many more charities - these are two that I believe have the highest chances of actually reaching civilians in Gaza and not being diverted.