Good question about the UN
As far as I understand, the UN is the main channel to provide aid working alongside the Red Cross and other groups. They provide security for aid trucks
But in terms of peacekeeping and peacekeeping troops and why not in Gaza, I suspect they are stretched too far in other conflicts? I had to look it up but "Specifically, they operate in countries like the Central African Republic, the Democratic Republic of Congo, South Sudan, Western Sahara, and Abyei. Other active operations include the Golan, Lebanon, Cyprus, Kosovo, and India/Pakistan, according to the United Nations.
I'm not sure, even if they had the means, how UN peacekeeping troops could be involved without becoming targets for both Israel and Hamas?