I don't know if it's a sin to gossip for christians or not, but I do know that is the most common reason I hear from christian friends and acquaintances for their not attending a church. They say they don't like all the backbiting and gossip. I also know if the gossipers were Heathens they would be persona non grata fast. We do not discuss another person's personal business with third parties. It is a violation of trust, which is extremely dishonorable behavior, and has no place in a faith family, be it a Kindred, Hearth, Temple or Church. If it wasn't told to you directly it is also none of your business and the offender (person trying to tell you the gossip) needs to be shut down immediately and told their behavior is completely unacceptable.
I call people who drive others from church with their own reprehensible behavior and the disgust inspired by their attempting to rationalize it ("well they were sinning first/too!" or somesuch) "Hypochristians". The term has come in handy many times since my friend Sylvi first introduced me to it years ago. Feel free to use it as necessary.