The one thing that I would add is to NOT place the person wearing the gag flat on their back. Drooling is inevitable and this position can make swallowing and breathing tricky.
Let me add to what the ladies have said. Not NO but HELL NO!!!
If you want to ignore your sub at least stay in the room reading a book and checking on her/him visually every few minutes. Even pretty well bound, any one can manage to get into a dangerous situation, when they struggle against the bondage.
The idea of leaving someone gagged, which makes breathing and swallowing difficult, scares the crap out of me. I learned a long time ago if you break your toys you can't play with them again.
If you kill them it tends to diminish your standing in the BDSM community and gets the law enforcement folks all riled up.
I've been involved in these kinds of activities for over 25 years and have never had to take a submissive to an Emergency Room.