First of all, I think setting up a situation where a sub is unable to use a safe word (unless that is pre-negotiated) is unprofessional. On the other hand, saying ouch or stop might not get you any relief. That is what safe words are for.

Gags have nothing to do with safe words or consent. I can still understand the word "yellow" or "red" with most gags. If I know I can't, then some other safe mechanism is used - tapping, specific grunts, etc.

I enjoy using gags when I want to make the sub feel more out of control or if I am just tired of his ceaseless comments. One of My favorites is to make a sub "self gag". He has to hold something in his mouth. If he drops it (other than to use a safe word) there are consequences.

In general, I don't use gags a lot. I like the verbal interaction with subs in session. Most of the time . . .