A GP-100 is actually 1 or 2 oz lighter than a comparable S&W 686 and will fit in the same holster and uses the same speed loader. Some guys seem to think they are big heavy monsters when they are not.
In my hand, the 686 in an equivalent barrel length has always
felt lighter and more lively. Maybe it's the grip / frame shape, or maybe the S&W's center of mass is a little closer to the rear. I dunno, I just know I don't want any more muzzle heft than either in a 4" barrel.
As for a some guys thinking they are a big heavy monster, I suppose that depends on what you are accustomed to. If your favored gun is a Ruger SRH or a Desert Eagle, I can see where the GP would feel lithe. Then again, if you prefer the older Ruger Six Series and S&W K Frames (as I do), then the GP (and 686 for that matter) both feel a little clumsy.
So for me, the pluses and minuses for the GP are:
+: Excellent bang for the buck, new or used. Durable, and serviceable (cylinder bolt notches over the chambers, non turning ejector rod, etc). Easy to disassemble. Soaks up recoil.
-: Ugly as sin (I know, beauty is in the eye of the beholder, but they are darn ugly to my eyes
). The billboard, need I say more? Though usually smooth, the triggers often exhibit more stacking and a more mushy return than I prefer. Lastly, the "strength to massiveness ratio" is IMO (for my preferences), not as optimal as the older Six line. The older Sixes are nearly as durable, but are much bulker than a S&W K Frame.
But YMMV.