There just too big and heavy.
I'm an avarage size guy and carrying a 4 5/8" Blackhawk or a 5 1/2" Redhawk IWB (both MUCH bigger and heavier than a GP-100) isn't hard IMO IF
you are using a high quality holster and a good stiff gun belt. I sometimes forget they are even there. Sitting here wtih a 5 1/2" Colt New Service IWB as I type.
There's no reason someone of average size can't carry a mid sized 4" GP-100 with relative ease in a decent holster/belt combo.