It's probably worth saying that we here at Shmoop feel kind of bad for Detective Gilpin, mostly because he's stuck playing second fiddle to The Unstoppable Rhonda Boney for the whole novel. Clearly he's the Watson to her Holmes, the Barney to her Fred Flintstone, and worst of all, the Garfunkel to her Simon. Jim Gilpin's chief function in the book is to be Rhonda's sidekick, and he's a flat character if we've ever seen one.
If you don't believe us, go back and look at the appearances he makes in the chapter summaries. Aside from making some inappropriate remarks about the pornography in Nick's man cave woodshed (45.40,42,44), Gilpin spends most of his time doing menial tasks, like making phone calls and demonstrating how the position of the ottoman in Nick's living room indicates that the crime scene was staged. He's just not a memorable person—but Rhonda Boney, on the other hand, is. So his relative flatness makes her alpha-woman personality pop off the page that much more.