I think that it's important to note that gossip implies a malicious intent to defame, slander, or spread libel (depending on the medium being used). To discuss the false doctrine or prophecy of a pastor, teacher, or prophet (Biblical appointments - 5 fold ministry in Hebrews) in a means to warn other Christians is not gossip. It is dealing with straight facts regarding what the individual taught or said and comparing it to scripture. In fact, not warning would be a demonstration of a LACK of love on the part of a believer to another person.