With a gun as beefy as the GP100, it will make .357 loads pretty tolerable to shoot. .38 special will have minimal recoil.
.38 special would probably be a good place to start as far as practice goes. .38 special +P ammunition would be a good choice for home defense. Should the gun ever be needed for defense, a .357 will go through the person and probably into and out of the wall behind them. Something to consider if there are other people in the home.
A 4" or a 6" would be a good choice,the 4" is the GP100 I would get.
Another thing you might consider for your concealed carry as your thinking about an airweight j-frame is the Ruger LCR. I was kind of skeptical about the whole polymer framed revolver at first, but it is a few ounces lighter than an Airweight, has the same capacity and the trigger on them is considered by many to be one of the best stock revolver triggers.