Thank you for the explanation Gabriel!

Both have the same meaning, however gossip-monger seems like it has more intensity to it, and the person who is a gossip-monger actually spreads other people's information in exchange of money since the word monger denotes a dealer or trader in a specified commodity, like a warmonger, for example. And by intensity I mean that the person might not do it for money, but is a compulsive gossiper.