I bet it is no more difficult to conceal than a S&W K frame (which I do) or L frame (which many do).
K&D Holsters would be one place to check. I bet most makers who do IWBs have the GP100 among their molds.
ETA: I just checked Kevin's website and he does list the 3" GP among the list of guns available in the Cochise Defender. If yours is a 4", I would call and ask if he can do one for it.