The Gospel Acclamation is the “alleluia” we sing between the Second Reading and the Gospel at Sunday Masses. During Lent we do not sing “alleluia” but a different acclamation such as “Glory to you, word of God, Lord Jesus Christ.” At weekday Masses, the gospel acclamation comes between the responsorial psalm and the Gospel. Here, “the Alleluia or the Verse before the Gospel, if not sung, may be omitted” according to the General Instructions of the Roman Missal (GIRM #63.c).
Recently, I asked a group of people here at SLM when the Gospel Acclamation is supposed to be sung. I received such answers as: 1) when the priest stands; 2) when the deacon stands and receives a blessing by the priest; 3) when the incense is brought up; 4) at some time after the Second Reading. The singing of the Gospel Acclamation accompanies the procession of the Book of Gospels to the ambo, much like the “Lamb of God” is sung as soon as the priest begins breaking the host (Fraction Rite) after the Sign of Peace. The Gospel Acclamation, “an acclamation of this kind constitutes a rite or act in itself by which the gathering of the faithful welcomes and greets the Lord who is about to speak to them in the Gospel and profess their faith by means of the chant (GIRM 62).
This is why we start the gospel acclamation as soon as the deacon or priest raises the Book of Gospels at Sunday Mass, to emphasize why we sing at this part of the liturgy.