Mistress Sidonia has put together a very comprehensive list of gag types, spread across a pair of posts here and here. It’s a handy reference for anyone looking for new toys or techniques to try, as well as those who like to go through lists ticking off what they’ve tried to date.
One thing that always amuses me about gags is how many dommes love the drool they produce. There may be some dommes out there who don’t care about gag drool, but if there are, I’ve never met them. I’ve seen plenty of dommes complaining about submissives slobbering over their feet or dribbling onto their outfits, but somehow gag drool is never an issue. It’s either hot, funny, cute or all of the above.
This image is by Joachim Baldauf for Paper Planes magazine.
This was shot by Rope Geek, whose tumblr seems to be deactivated. Models are
I found this on the 
Based on the image signature, this is by the artist 



I believe this is from the