Introduction: Deuteronomy (Dvarim) 1:7 as translated by Sefaria states:
Start out and make your way to the hill country of the Amorites and to all their neighbors in the Arabah, the hill country, the Shephelah, the Negeb, the seacoast, the land of the Canaanites, and the Lebanon, as far as the Great River, the river Euphrates.
Sifri on that verse states that the seacoast mentioned in that verse includes Gaza. That is to say it is on the list of the regions that the Torah commands the nation of Israel to possess in accordance with the rules set down in Numbers (Bamidbar) chapter 33.
By pressing on the link you are invited to listen to an 11:36 minute, audio talk by Rabbi Moshe Kaplan on Gaza being part of Eretz Yisrael
original source from: https://meirtv.com/en/eyc104-parshat-yayera-gaza-is-ey-land-for-peace–9396l
The Section below is not part of Rabbi Kaplan’s talk but is based on Rashi
Breishit / Genesis Chapter 15 verses 5 to 21 – G-d Promises the Land of Israel to Avraham (as translated by Sefaria)
He took him outside and said, “Look toward heaven and count the stars, if you are able to count them.” And He added, “So shall your offspring be.”
And because he put his trust in the LORD, He reckoned it to his merit.
Then He said to him, “I am the LORD who brought you out from Ur of the Chaldeans to assign this land to you as a possession.”
And he said, “O Lord GOD, how shall I know that I am to possess it?”
He answered, “Bring Me a three-year-old heifer, a three-year-old she-goat, a three-year-old ram, a turtledove, and a young bird.”
He brought Him all these and cut them in two, placing each half opposite the other; but he did not cut up the bird.
Birds of prey came down upon the carcasses, and Abram drove them away.
As the sun was about to set, a deep sleep fell upon Abram, and a great dark dread descended upon him.
And He said to Abram, “Know well that your offspring shall be strangers in a land not theirs, and they shall be enslaved and oppressed four hundred years;
but I will execute judgment on the nation they shall serve, and in the end they shall go free with great wealth.
As for you, You shall go to your fathers in peace; You shall be buried at a ripe old age.
And they shall return here in the fourth generation, for the iniquity of the Amorites is not yet complete.”
When the sun set and it was very dark, there appeared a smoking oven, and a flaming torch which passed between those pieces.
On that day the LORD made a covenant with Abram, saying, “To your offspring I assign this land, from the river of Egypt to the great river, the river Euphrates:
the Kenites, the Kenizzites, the Kadmonites,
the Hittites, the Perizzites, the Rephaim,
the Amorites, the Canaanites, the Girgashites, and the Jebusites.”
Rashi: In the Merit of the Sacrifices They Will Remain (or be established) In It
Rashi to Breishit / Genesis 15:6
‘והאמין בה AND HE BELIEVED IN THE LORD — He did not ask Him for a sign regarding this; but in respect to the promise that he would possess the land he asked for a sign, inquiring of God, במה אדע “By what sign shall I know [that I shall possess it?]” (Genesis 15:8).
ויחשבה לו צדקה AND HE ACCOUNTED IT UNTO HIM FOR RIGHTEOUSNESS — The Holy One, blessed be He, accounted it unto Abraham as a merit, because of the faith with which he had trusted in Him. Another explanation of במה אדע is: he did not, by these words, ask for a sign regarding this promise that he would possess the land, but he said to Him, “Tell me by what merit they (my descendants) will remain in it (the land).” God answered him, through the merit of the sacrifices (Taanit 27b).