I'm going to tell my wife's new boyfriend's girlfriend that she is being cheated on
Does your wife know about his girlfriend?
No, I've heard that they are dating behind my wife's back.
It is understandable not liking that your wife has a boyfriend and you want to get back at them by telling the boyfriend's girlfriend about the relationship. But if your wife has moved on, then you should not interfere with her relationships and not do it for revenge either.
But I could get a fist bump for doing this.
It sounds like your relationship with your wife has ended and that she has moved on.  If you suspect that there is some infidelity in her new relationship, your wife should be the one that you tell.  However, it is not your place to break up this new relationship, that would be their decision.