Many people in my parish sing all of the Gloria. (In the Spanish service, we have the complete words for the arrangement we're using on the song-sheet handout.)
At English Masses, you can often tell who the visitors are because they turn around to stare at those singing all the Mass parts, or harmonies to the hymns. Phooey, I say.
LOL!!
You should see the looks on the visitors faces when we do the Kyrie, the Gloria, the Creed and the Sanctus in the respective liturgical languages and no one picks up a guide.
Or when we all break into the Prayer to St. Michael after the final blessing.