There are many different types or styles of gag bits. There are full gag, sliding gag, partial gag, and as many different styles of mouth pieces. A lot of people love them and use them, and a lot don't like them at all. I am in the middle. I DO NOT use them on a young horse!! I only use them on more finished and seasoned horses who may need a little "wake up" or lifting. I only use the snaffle mouth piece, no twisted wire, no tiny mouth pieces either. I have a thick snaffle on mine, with no wire, etc. headstall with it. I use it when a horse is heavy in the mouth, heavy on my hands, or heavy upfront and won't lift up its shoulders and front end, or lift up off my hands. I don't use it all the time either, just here and there as they need it to get them up off their forehand. That is really what a gag is best used for, to bring up the front end, and that is the action you get from the bit when you pick up and put pressure on the bit. A traditional snaffle is fixed and only allows you to pick up to its stopping point, so you kind of max out on the pressure value. A true sliding gag bit continues to pull up in the horses mouth as far as you want to pull it, which is not a good idea either, a lot of damage can be done with a gag bit, and in my opinion is only for professional use or for someone who really knows what they are doing and can feel their horse, so as to know how far is to far and to stop before they can damage the horses mouth. So, I hope that helps, I know its a novel....sorry. I really have only had two horses I've ever used the gag with, and both were long backed, "heavy" horses who didn't want to elevate and pick up their shoulders, and I tried a lot of other things prior to just putting a gag in, it was like my last resort.
So really, it can be as harsh as the person holding the reins, and is meant to bring the horses front end up, and give you more shoulder, front end elevation and control. A safety note......don't snatch or pull abruptly up for you can flip them with a gag that way.
Now, for running events, such as barrels I know a lot of people use them and swear by them. For when you have that much speed the gag helps to grab the horses shoulders for the turns and get them sitting back on the hips. I see how it can be a benefit for the running events, for when you add that kind of speed your reaction time from the horse when you pick up the shoulder and turn is delayed, the gag helps to get a "right now" response from the horse. But again, I would go with a partial gag, so there is a stop to how far up the bit is going to go before it has a stop point, or a combination gag, such as the Myler short shank combination bridle( I do use that on all my babies!!) It has a partial gag with fixed "stop points" for the bit, as well as nose and chin pressure when you pick up on the bit. I love this bit!! Again, in a snaffle mouth piece.
So I hope that answered your question, I didn't intend to give such a long response.