The forcing cone on a GP series is slightly larger than that of an L frame if I recollect correctly. They cylinders of the GP series and the L frame are the same size. Depending on the holster and how closely it's formed I can fit an L frame in some of my K frame holster and have relegated one of my L frame holsters for an N frame.
I don't disagree that the GP is a more robust gun than the L frame. It took me a long time to admit that publicly.