gpp = grains of arrow weight per pound of bow holding weight as in 44# holding weight and a 470 grain arrow equates to 470/44=10.7gpp
gpi = grains of weight per inch of arrow ... however, this is mostly used in determining the bare shaft weight when choosing a final weight finished arrow for a specific bow and task. a raw 220 grain carbon shaft that's 32" long results in a value of 6.9gpi. i need to qualify this a bit more - gpi can help if you have to choose between two shafts that both have the same stiffness/deflection, but different mass weights. each will make up an arrow that will spine properly for a given bow, but the mass weights will be different. this can be a deciding factor for the game hunted and the shot distances encountered. in example, one might want to shoot a faster arrow for pronghorns than for whitetails, and therefor a lower gpi arrow might be the better choice.
i will add that playing the numbers game, as in above, is nowhere near as important as whatever works best for your bow, the game you hunt, and you. there are exceptions to every rule of thumb.