I really should know this
I think (but I'm a bit fuzzy on this) that the gloria isn't usually said or sung during advent(four weeks up to christmas) and lent. I don't know if exceptions are made for weddings or not. Of course, that may be totally wrong....
I think the change during lent where you don't say the Christ has died, christ has risen.... I don't think it affects the Gloria at all. I work with a priest so I will check for you tomoro for sure!
During lent you dont sing the Alleluia. Im not aware of anything different about the Gloria, but I could be mistaken.
The Gloria is rarely sung at weddings, though.
It was sung at ours, I still get oosebumps thinking about it and watching it on the DVD. I thought people would complain because the music made everything a bit longer but anyone who has mentioned the music said how wonderful it was.
I'm fairly sure its still said during advent