It is logical to assume, even in the Gaza strip, that there are some places less dense than others. And yeah, there is "countryside" and there are smaller towns & villages.
Gaza Strip Population Density - OCHA map - Question of Palestine
Your point about how about supplies & basic needs is a fair one. I don't know what's available or not but clearly it'll be inadequate.
Also fair enough about my comment re: West Bank. It's not practical to go there now. But sure, if I was in Gaza prior to this conflict, I'd be trying to relocate to the West Bank.
Bottom line to me. If you knew your city was going to be attacked, your enemy is telling you to leave, and there was going to be a lot of street to street fighting (and we all know this don't we?), the first priority is to get out of that city.
If I was in that situation, I'd probably head to the outskirts of the city (if I thought it was relatively safe from the fight) or south towards Egypt where very likely there'll be some sort of humanitarian assistance.