John Gill Commentary Psalms 118:20

"This is the gate of Jehovah; The righteous shall enter into it." — Psalms 118:20 (ASV)

This gate of the Lord, into which the righteous shall enter . This seems to be spoken by some other person or persons, distinct from David and the Messiah, pointing at some particular and principal gate, upon hearing the above word:

by which is meant, not the gate of the house of the sanctuary of the Lord, as the Targum; but the Messiah himself, afterwards spoken of as the stone rejected by the builders, and made the head of the corner; who is the way of access to God; the door into the church or sheepfold; the strait gate that leads to eternal life; by which none but righteous persons enter into heaven; even such who are made righteous, through the imputation of his righteousness to them; see (John 10:1John 10:9) (14:4) (Matthew 7:14) .