Who is Gilly Gilbert from The Guernsey Literary and Potato Peel Pie Society and what is their importance?

Gilly Gilbert is a writer for the "London Hue and Cry" who gets into an argument with Juliet when he asks her about leaving Rob Dartry at the altar. He is embarrassed by the news reports about the incident and plots revenge against Juliet and Sidney with the aid of his mistress, Billee Bee.