Sorry about your GP100. The GP100 is about the strongest mid frame revolver going. But shooting the 125 gr "flame thrower" will pound any revolver to death and will erode the forcing cone on the inner edges and cause the crack at 6 oclock.
Here is a picture of a GP100 with a cracked forcing cone. The fella with threw 100 rounds a week for a year. The result a cracked forcing cone. Ruger replaced the barrel. Stay with the heavy grain rounds and I doubt that this will happen again.
Good luck,
Howard
This is what he was using as a load. Factory 125 gr won't be as hard but still will pound the revolver.
125 grain Hornady XTP over 22 gr. of H110 with a Remington 5 1/2 primer.