From the always-superb commentary of Dom Johner in The Chants of the Vatican Gradual:
Today the Introit, Gradual, and Communion speak of Sion, i.e., of Jerusalem. The Alleluia verse also alludes to this. For at Rome the principal service was held in the Church of the Holy Cross in Jerusalem, close to the Lateran. Formerly it was a royal palace; now it shelters a most venerable relic of the holy cross. Our present Sion is the Catholic Church. It is also our individual soul, and likewise the church building in which we look for the Redeemer today. Here it is that we are being prepared for the heavenly Sion.
the Lord will come to save the nations;
and the Lord will make his glorious voice heard,
to the rejoicing of your hearts.
V. Shepherd of Israel, hear us,
you who lead Joseph like a flock.
(Isaiah 30:19,30 / Psalm 80:1)
… One seems to hear [in the opening phrase] a herald proclaiming to the people of Sion the most important news ever told, the tidings which mankind had been awaiting for centuries. The messenger commissioned by the Lord Himself, would have this message of joy penetrate into all hearts: "The Lord shall come to save the nations." And you yourself may listen intently for the voice of the Lord. For He will speak as one who has power; He will speak of His grace and transcendent truth and glory. His voice will cause the heart to overflow with joy.
Where such great things are promised, the petition of the psalm verse comes to mind spontaneously: "Give ear, O thou that rulest Israel." Help us to live ourselves into this season of grace. For most lovingly didst Thou lead Joseph from imprisonment to the regal throne. The words of the antiphon were verified when the Lord came. Joy filled the hearts of the shepherds when the Lord, through His angels, announced to them the message of peace. And although the Child of Bethlehem could not at that time speak a word, He has often conversed secretly with our souls in laetitia cordis. A day will come when His voice will resound majestically over the millions of men who have ever inhabited the earth, announcing eternal joy to them who have listened to it during their lifetime. …
[T]he fifth over ecce fixes the attention. And then it is as if the Lord Himself slowly and solemnly came into view. Hitherto He had sent the prophets; now He Himself appears. He comes not to judge, however, but to redeem; He comes to bring redemption to all the nations. This thought is given a more detailed treatment in the Epistle; and in the Gospel the Lord speaks of His activity: "The blind see, the lame walk, the lepers are cleansed, the deaf hear, the dead rise again, the poor have the gospel preached to them." He comes to redeem the gentiles. At this gracious manifestation of divine favor the melody bows down in gratitude. …
Out of Zion his perfect beauty shines forth.
God is coming in broad daylight.
Summon before him the consecrated nation
who made a covenant with him by sacriﬁce.
(Ps 49:2-3, 5)
When the Lord will come, He will bring joy to the hearts of men: that is the promise of the Introit. The Epistle closed with similar words: "Now the God of hope fill you with all joy and peace in believing." This thought is prolonged by the Gradual. It speaks of the beauty of Him who is to come. All beauty, and especially that of the coming Messias, cannot but produce joy: that is the theme of this bright song. Not infrequently does it remind us of our modern major scale. In other passages the Psalmist has painted the beauty of the Messias in brilliant colors, describing Him as the most beautiful of the children of men. Now He is to come—and manifeste, in visible form. But this Ruler will not isolate Himself from His subjects, as is the custom of Oriental sovereigns. He will show Himself, and with the magic of His beauty He will capture all hearts.
But he does not come alone: a great host accompanies Him. Of this the Gradual-verse speaks, as we also read in one of the Advent antiphons: "Behold, the Lord cometh and all His saints with Him." When He comes at Christmastide, the saints who have sealed the covenant with the sacrifice of their blood will surround His cradle; St. Stephen, St. John, the Holy Innocents. But the full grandeur of these words of the Gradual will be realized only at the end of the world. When the angels' trumpets will sound—some persons will perhaps hear their echo in the notes of Congregate—then will arise both the wicked and the just, the saints who sealed their covenant with God by sacrifice, by loyalty to the end, frequently by a martyr's bloody death. Now they all come to form the radiant retinue of the Saviour. However enchanting this prospect may be, God, the eternal Sun, infinite Beauty, of whom the saints are but reflections, will appear infinitely more glorious and resplendent. …
Hear the soft yet persistent undertone: Gather ye around Him, all ye His saints! Let us be mindful of our vocation to aspire to sanctity, since we are privileged to assist at the holy Sacrifice of the Mass. Here we should renew the covenant with Christ which He sealed with His bloody sacrifice.