I do not participate in Gang Bangs, nor is it a fantasy of my Mr.'s but I can empathize with your wife. I think a Gang Bang is a different kind of swinging. To me it is more of a fetish, and although your wife may appear to be disrespected during the bang, she is probably getting a lot more out of it then she will ever tell you. Although these men are ravaging her, she is the ultimate center of attention. And she has the ability to be very, very naughty in the process. She is getting a different kind of respect from these me. It is not societal respect, it is respect from the standpoint that everyman wants her, and everyman there wants to play with her, and every man can play with her. I think she is actually in the position of power more than anyone else there. I would imagine that can be intoxicating.
Now that you know why she wants to do it..... you need to figure out why you don't want her to do it. I read your post a few times, and the only thing I can figure out is that you are uncomfortable with it because it is not your fantasy anymore, and your wife hasn't met your comfort level by helping establish rules for the bang.
I still think that there is more to it than that. What exactly makes you uncomfortable? What exact activities during the bang would you like for her to avoid? Are you having any jealousy issues with the other men because she is becoming more open to other acts? (Like Blow jobs and Anal Sex)
Do you have a hard time respecting her because she is acting, in your opinion, slutty? Are you feeling left out?
Now in our relationship, if Mr Indy asked me not to do something I would stop immediately and be with him. We have a good set of rules for almost any circumstance, and we are both very comfortable with them. To a certain degree I can understand where your wife is coming from. It sounds like from what you have written so far, that you guys had some rather liberal rules to begin with, and since you have been in the lifestyle for more than a decade, I think that is reasonable. But now, it seems to me you want to make changes to the rules, that maybe didn't exist before, or without reason. I am not advocating that there has to be a reason for every rule change, but I do think that since this is such an important issue to you, and clearly an ongoing issue between you two, you need to have reasons.
If I were your wife and truly enjoyed something, and you allowed me to do it for a period of time, and then all of a sudden didn't want me to do it, or wanted me to restrict it, I would be confused and frustrated, and would probably behave much like your wife is doing.
To me it is unreasonable for you to impose rules this far into the game without valid reasons.
So again, my advice to you, is for you to determine what it is exactly that you do not like about her GB and to articulate that in a compassionate way to your wife. You then must sit without judgement and listen to her explain what exactly she likes from the GB. Hopefully when the 2 of you compare notes, you will be able to reach a compromise that serves both of your needs.
At this point you are in a stalemate, and neither one of you are going to get any farther along without good, honest communication without judgement or hesitation.