1)
Why is it called "ha'Sha'ar la'Shem"?
1.
Rashi: Refer to 118:19:1:1.
2.
Radak: The praise is proper only for Him.
3.
Malbim: The king had a special gate via which he entered Beis Hashem.
2)
Will only Tzadikim enter the gate?
1.
Radak: They will praise Him for all that He did with them, like I do.
2.
Malbim: Normally, this gate was only for the king. Now, he permitted Tzadikim to enter with him.