The Gloria is sung or recited on Sundays during the liturgical year, *except* during the seasons of Advent and Lent.
Therefore, you would typically sing the Gloria on these Sundays:
* Ordinary Time: All Sundays during Ordinary Time.
* Christmas Season: All Sundays during the Christmas Season
* Easter Season: All Sundays during the Easter Season.
Keep in mind that the Gloria is also sung at Solemnities and Feasts, even if they fall on a weekday, as well as at special celebrations.
It's always a good idea to consult the specific parish bulletin or ask the music director for clarification, as there might be slight variations depending on local customs.