According to the General Instruction of the Roman Missal, "The Gloria is an ancient hymn in which the Church, assembled in the Holy Spirit, praises and entreats the Father and the Lamb. It is sung by the congregation or by the congregation alternately with the choir, or by the choir alone." (#31)
The Gloria, originally composed in Greek, belongs to that body of early Christian hymns written by private individuals and modeled after the scriptural psalms and inspired by the song of the angels at the birth of Christ. It is also known as the "Angelic Hmn" or the "Greater Doxology" as contrasted with the "Lesser Doxology" which concludes the Eucharistic prayer. The text first appeared in the East where it was sung during Morning Prayer. In the West it came to be used at Christmas and only by the bishop. In time the humn was sung at all SUnday and martyr feast day liturgies at which the bishop presided. Only during the eleventh and twelfth centuries was its use extended to all priests. The ritual of Sunday Celebration of the Word and Hours promulgated by the Canadian bishops recommends that this humn of praise "is particularly appropriate during the Christmas Season, solemnities and during the Easter Season." (#50)
The presence of the Gloria among the introductory rites results in a certain degree of tension. Being a hymn, it should be sung. And yet singing the hymn is to lengthen further an already overly extended rite of preparation. Also, singing another hymn so soon after the opening hymn of the Liturgy and the singing of the Kyrie, could over burden the assembly and obscure the importance of the proclamation of God's word. Moreover, the frequent use of the hymn tends to rob it of its festal character. This has led many liturgists to remark that it would be advantageous if the Gloria were required only for very special feasts and occasions. The ritual of Sunday Celebration of the Word and Hours further suggest that the Gloria could be used during the procession and enthronement of the Lectionary or as a post-communion hymn of praise providing it is sung outside of the seasons of Advent and Lent.
[Source: St. Paul Roman Catholic Parish Bulletin, Saskatoon, SK, Canada, March 2, 2008]