During the Shmitta year we were allowed to eat fruits and vegetables from Gush Katif. Therefore it was not considered part of Eretz Yisrael then. Why are rabbis now calling it an integral part of the land of Israel?

There is a difference between the 'Land of Israel' which is within the borders of the land as written in the Torah - reaches south until the 'Egypt River', which some opinions hold it to be Wadi El Aris, while most of the others say it's the Nile (Gush Katif inside at any rate), and the area that has been sanctified, or not, for the matter of Mitzvot Htaluyot Ba'aretz (such as Shmitta), such as Gush Katif and the east bank. Rabbi Ro'i Margalit