Should I tell my girlfriend that I know about a secret group chat conversation she's in?
What did you do to find out about this information?
I found out by reading her messages.
Reading the messages of your girlfriend is a very untrustworthy thing for you to do. If you do not trust her than you should just confront her and ask about the information you want to find out.
I don't think that can really happen. I'm gonna tell her and I bet we will break up because of it.
She really may want to break up, since it will be obvious that you betrayed her trust and read her messages. You should really consider being more upfront and honest with her when you want to know something in the future. I will help to build a strong relationship.