the gloria is an ancient trinitarian hymn. originally this hymn was used as a canticle in the morning office along with the biblical canticles. because it invokes the song of the angels at the birth of Christ, it the pope began using it long ago at the christmas mass, and from there it spread to be used by the western church generally at any more festive mass outside of the seasons of advent and lent. at the same time, it stopped being used in morning prayer in the roman rite, because of a growing preference for the use only of biblical canticles.
in the eastern church it has never stopped being used at morning prayer, and some modern revisions of the office (including that of the episcopal church) use it there. in the episcopal church it is sung at morning prayer on thursdays except in the seasons of advent and lent. it is also used at the eucharist, particularly in the seasons of christmas and easter.
glory to God in the highest,
and peace to his people on earth.
Lord God, heavenly King,
Lord Jesus Christ, only Son of the Father,
for you alone are the Holy One,