Q. 1. What is the purpose of the Gradual during the Holy Mass?

A. 1. The Gradual (Latin: graduale or responsorium graduale) is a chant or hymn in the liturgical celebration of the Eucharist in the Catholic Church, and among some other Christians. In the Tridentine Mass it is sung after the reading or chanting of the Epistle and before the Alleluia, or, during penitential seasons, before the Tract. In the Mass of Paul VI the gradual corresponds to the Responsorial Psalm. There is the option to replace this psalm with the gradual, but its use is extremely rare outside monasteries. It is part of the Proper of the Mass.

The Gradual, like the Alleluia and Tract, is one of the responsorial chants of the Mass. Responsorial chants derive from early Christian traditions of singing choral refrains called responds between psalm verses. According to the Catholic Encyclopedia, it (and the associated Alleluia or Tract) is the oldest of the chants of the Proper of the Mass, and, in contrast to the Introit, Offertory, and Communion, the only one that was not sung to accompany some other liturgical action, historically a procession. Until about the fifth century, it included singing a whole psalm. They were sung in the form of a psalmus responsorius, i.e. the whole text was chanted by a reader appointed for this purpose. For some time before Pope Gregory I, to sing these psalms was a privilege of deacons at Rome, a privilege he suppressed in 595. The people answered each clause or verse with an acclamation. This apparently dates back to the synagogue tradition, and can even be seen in the structure of some Psalms (such as 136|135). Originally, there was a psalm sung between each reading, of which in the fifth century there were three (Prophets, Epistle, and Gospel).

When the Old Testament reading was later dropped, the other two psalms became the Gradual and Alleluia, ordinarily sung one after another, until the 1970 Missal restored the three readings on Sundays and Solemnities.

The modern Gradual always consists of two psalm verses, generally (but not always) taken from the same psalm. There are a few Graduals that use a book of scripture other than the Psalms (for example, the verse for the Feast of the Immaculate Conception is from the Book of Judith), or even non-scriptural verses (for example, the first verse in the Requiem Mass).

The Gradual is believed to have been so named because it was sung on the step (Latin: gradus) of the altar, or perhaps because the deacon was mounting the steps of the ambo for the reading or singing of the Gospel. However, early sources use the form gradale ("graded" or "distinguished"), and the Alia Musica (c. 900) uses the term antiphona gradalis for the Introit.

[Source: https://en.wikipedia.org/wiki/Gradual ]