Well, since Advent is over, the Gloria SHOULD have been said. However, there are at least two major possibilities:
ONE: The priest simply forgot. It happens. If next week everything is fine, and the Gloria is said, that is probably what happened at today’s Mass–simple human error. After four weeks of NOT saying the Gloria, it is not unlikely that the priest forgot.
TWO: The priest chose to omit the Gloria–for whatever reason HE thought suitable. Perhaps his homily was going to run over, perhaps he was going to “cover the matter” in the sermon and thought the Gloria was “overkill”, perhaps he was poorly prepared in seminary and thinks (because of the Lenten and Advent practices of not saying the Gloria) that the Gloria is “always” optional. . .etc. etc. If that is the case, then the Gloria will be more frequently omitted. Then you might consider talking with the priest, saying that you had understood from (the GIRM --print a copy) that the Gloria was always to be said, and wondered if he realized he had not said it on (list the Sundays). Then, listen to what he says. If he tells you that the bishop says it’s optional, or that the GIRM is suggestion only, or (heaven help you) that he is sorry you are the “Church police”. . .approach the bishop, or higher if necessary. But chances are that, if he is approached respectfully, that he will tell you that he will check into the matter, that he will say, “gosh, you know, thank you for bringing this to my attention, I’ll be sure to say it appropriately from now on”.