Another vote for the 6 inch barrel. The GP100 is a beefy revolver to begin with. I have shot the 6 inch version on several occasions (my brother's) and it is a nice, solid, accurate firearm. Nothing against the other barrel lengths as I haven't had an opportunity to shoot them, handle them at LGS yes but shoot, not yet. They are pretty though.
I am just going to throw this out here but I own the SP101 2+ inch barrel that handles full sized loads well. I believe there is a 4+ inch version produced now. Being the GP100's little brother you do lose some of the frame mass and one round. If you are looking to conceal I would recommend something smaller than the GP100 in general but then again I am built like a mantis so my conceal choice is different than some folks lol.
At the picture-posters - Ruger porn is always appreciated