I'm with Friend for the most part. I don't think GPP is a good measure unless you stick to a very narrow range of bow weights.
With a low weight bow, say 40lbs, you need a higher GPP to try and preserve all the momentum you can. With a higherweight bow, say 75lbs, keeping every slug is not as critical, so you can get away with a wider range of GPP.
I start at around 450 grains of arrow for a 40 pund bow and add 50 grains every ten pounds or so of bow weight. Makes a lot more sense than saying a 450 grain arrow will work out of a 45 pound bow but isn't going to penetrate out of a 55 pound bow.