I found out my GF has been cheating on me for over a year. A few different guys, and one is married, and they've been hooking up for over a year. I want to to rat him out to his wife, and bust my GF. But I don't want her to know I looked at her phone?

What she did was WAY worse than you looking at her phone, which really isn't a big deal. She is in the wrong here. Telling this guy's wife may be a little drastic but if it's possible, you could talk to him and get him to see that what he has done is wrong and maybe he can come clean to his wife instead. Of course if you feel too betrayed by your girlfriend then it may be a bad idea to talk to this guy, in which case you should go to the wife. Try to remain calm and don't go in there accusing people. Just say what you have found on your girlfriend's phone and that you suspect they have cheated. Let her talk to her husband after that. 

I'm sorry that this happened to you. Best of luck in the future :) I hope you find someone who treats you better.

You know the right thing to do. Leave her and rat him out. Not worth your time and energy.