A transom is a small section above a door which lets in light or which can be opened like a window for ventilation as shown. The meaning of "over-the-transom" is "unsolicited", that is, submitted to a publisher for publication without prior contact, the idea being that a manuscript might be thrown through an open transom into a publisher's office in the hope that the author's work might be published. I am not familiar with a definition that has to do with gossip, but it makes sense.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.