The phrase "blow the gaff" means to reveal a secret or expose someone's wrongdoing. There are several synonyms for this phrase, such as "spill the beans," "let the cat out of the bag," or "break the news." These idioms all share the same connotation of sharing sensitive information that was intended to remain secret. Another similar phrase is to "drop a bombshell," which often refers to the revelation of surprising or shocking news. It's always important to be mindful of the information we choose to reveal and the potential consequences of doing so, especially when it involves someone else's privacy or reputation.