The Palestinians are descendants of the ancient population of the area. It is dishonest to question this.
Conquerors come and go, but they hardly ever mass depopulate the area: that would leave no-one to rule over and tax. Generally, they just take a place over and impose an aristocracy that is just a tiny proportion of the total population (e.g. the Norman invasion of England). There are identifiable cases of mass migration e.g. the Anglo-Saxon invasion of England, but even still modern studies show the genetic make-up of the English is more ancient Briton than Anglo-Saxon. The historical record does not support mass depopulation of Palestine at any point in the last 2000 years.
It's not just the historical record: genetic studies of Jews, Palestinians and other Middle Eastern groups readily vindicate the modern Palestinians as being indigenous to the area. Many Jews are very closely related to the Palestinians genetically, which makes sense when you consider their ancestors lived side by side 2000 years ago and where the diasporic Jews did relatively little interbreeding.
So Jews and Palestinians are very closely related, would you call the Jews settling in Israel an ancient community living on their ancestral land? Would you say the average Englishman is part of an ancient community because of their genetic makeup? Do you claim your land for your ancestors? If I moved to Ireland, do I deserve special land privileges because of my ancestry?
This isn't a racial question, people move. Something as simple as moving to a different house down the street is going to make you look silly if you start claiming it as the land passed down from your ancestors, but people move a lot further than that, particularly if you live in a place with an intense history of war. Like, you say conquerors hardly ever mass depopulate an area, but the region under Jewish and Christian rule ~2000 years ago had over a million people, and by the end of the Muslim conquests 80% of that population was gone. Do you imagine those who stayed and survived have very clean direct claims of lineage in specifically Gaza?
And then, of course, the same people who will try to tell you that the people of Gaza have been their for centuries, that the olive trees were planted by their ancestors 1000 years ago, and that the churches have been there even longer... the same people saying these things will immediately about-face and tell you that Palestinians are all in Gaza because they were kicked out of Israel by the Israelis in 1948, and that's why they deserve to retake all of Israel.