Look up Clint Smith he has some experience.
He routinely carries a revolver larger then your GP100.
I do not want anyone to talk you out of carrying you GP100, I have and still do carry larger handguns than this when it fits the task at hand and conceal them pretty well under a Columbia outdoors shirt when I'm hiking, or a Hawaiian shirt lol.
It's more about getting the right holster, belt, and clothing than the size of the gun.
I remember standing in GM and listen to the salesman pontificated that you couldn't carry anything larger than a G-26 or J frame. I smiled and asked "then if someone came in carry an N frame S&W you could tell right away?"
"You yeah !"