וַיִּזְרַ֤ע יִצְחָק֙ בָּאָ֣רֶץ הַהִ֔וא וַיִּמְצָ֛א בַּשָּׁנָ֥ה הַהִ֖וא מֵאָ֣ה שְׁעָרִ֑ים וַֽיְבָרֲכֵ֖הוּ יְהוָֽה׃
И засеял в этой земле Исаак и нашел в том же году сто крат; и Господь благословил его.
Rashi
בארץ ההוא IN THAT LAND although it (Gerar) had not the sacred importance of Erez Yisroel proper — i. e., as the territory of the seven nations, and is therefore not as fertile as the Holy Land (cf. Ketubot 112a).
Rashbam
בארץ ההיא, even though the soil of this land is exceptionally hard;
Radak
ויזרע...מאה שערים, 100 times as much as they had expected to harvest from this planting.