Does anybody know why David Gilmour didnt take part of that live?

Gilmour and Waters have never been friends.

After The Final Cut, things got even worse due to legal issues about Pink Floyd name and it's use after Waters departure.

Only "recently" (2005) they played toghether at Live 8, the last time before Rick Wright dead.

Gilmour played only once in Water's The Wall tour, as a special guest.

Currently, they both publicly state that there is no intention to regroup the band.

He only did one performance of Numb once in London. It's on youtube, and also part of the special features on the deluxe edition. It's excellent. Gilmore also did the last song, and Floyd's drummer was also on stage for that.

Having Gilmour appear and play on "Comfortably Numb" would be the only way to make this film better, but I would've been shocked if it had actually happened. Too much bad blood between them. It was nice that Nick Mason sat down for the Q&A session, though.