The Sign of Peace is indeed optional. I have stated this before and others have argued with me but they are wrong. If you look in your St. Joseph Missal provided in many parishes, it will say something like “If appropriate, the priest will now offer the Sign of Peace;” If appropriate being the operative words, meaning the whole thing is up to his discretion. A priest may not find it to be appropriate at a funeral a for example. In parishes where the Gloria is not omitted during Advent, the priest simply isn’t bothering to read and figure out what should and should not be going on at that particular time in the calendar.