1)
Why does it say "She'ar Yashuv"?
1.
Rashi: The remnant in them 1 will return to Hashem.
2.
Radak: In the days of Chizkiyah, most of Yisrael were exiled. What remains of Yisrael will return.
1
Malbim: Who survived the sword.
2)
Why does it say "She'ar Yakov"?
1.
Malbim: The entire remnant [will return].
3)
Why is He called "Kel Gibor"?
1.
Rashi: He showed to them His Gevurah against Sancheriv.