We mean “the Gloria” the prayer beginning “Glory to God in the highest and peace to his people on earth …”.
From the 2002 GIRM n. 53: “It is sung or said on Sundays outside the Seasons of Advent and Lent, on solemnities and feasts, and at special celebrations of a more solemn character.”
So generally it should not be said at Masses on:
– Monday to Saturday.
– Sundays in Lent and Advent
Except:
– feast days of the rank Solemnity or Feast
– “special celebrations of a more solemn nature”.
This “special celebrations” gives extra flexibility to the priest. My understanding is that outside of Lent and Advent the priest would be free to include the Gloria during the week. The special celebration may be a wedding, feast of the rank Memorial or Mass with a particulary large congregation.