You don't come out when the opposition is going wide and he has two defenders closely chasing him. Just watch the replay to see Buffon's hesitation and then making the wrong decision to come out. It's clear. We all love Gigi but it's the truth.
Also, it's clear that Buffon doesn't trust Ogbonna and Caceres in defense. Which makes him do some unusual stuff that he doesn't usually do when Barzagli and Bonucci are on.
His mistake IMO is actually previous to the goal. I mean, look at his "starting position". He's insanely wide and doesn't cover well the nearest post to Gomez. Out of position, I'd say. At that point, he's right to come out, closing him a bit of space, but it's still a pretty easy chance and a forward like Gomez can't have troubles to score that. However it was a nice action by Violamerda, a great assist and the stop/preparing of Gomez excellent. Matri would have probably chest-stopped it with his back to the goal and lost it-passed behind. Caceres should have done better, he loses big Mario's steps. I don't actually understand people blaming Ogbonna, the pass actually overpass him, it's too high for everyone to reach it, and Gomez is not his man. I'd add, it's remarkable that after the assist he does his best to repair Caceres' "damage" by trying to catch Gomez even if he's obiouvsly too much behind, and not his fault.