During Advent (as during Lent), the Gloria is suppressed (omitted, if you will). However, Advent carries two exceptions: the Solemnity of the Immaculae Conception of the Blessed Virgin Mary and the Feast of Our Lady of Guadalupe. The Gloria is to be sung/recited during these two days, the former because it is a holy day of obligation, the latter because it is a feast in the United States (as well as a Holy Day of Obligation in Mexico).
Lent (depending on when the season begins and ends) carries with it a couple of exceptions: March 17th, the Feast of St. Joseph and March 25th, the Feast of the Annunciation of Our Lord). The Goria is to be sung/recited; however, the Gospel Acclamation cannot be Alleluia; it must be one of the Lenten forms. I may be wrong on this, but the Feast of the Chair of Peter might count as an occasion when the Gloria is sung/recited. This falls on February 22nd and is a huge feast at the Vatican, for obvious reasons. As I said, these dates are dependent on whether or not they fall during the Lenten season.