The problem with the overall GP is that it is a measure of your roster completeness, not the actual power of it. E.g. with all those LSB being rampant nowadays, it is easy to find players with over 7 millions GP without even a single GL, and that can be easily beaten by players with less than half that GP.
To match players with related GP, one option could be to compare also the mean GP of the X strongest toon of each roster. Where X scales up at each division level. But, even so, with all those new shiny toys being far more stronger than older teams, it can become unbalanced pretty quickly. To really fix things, CG should add a new counter that takes into account the actual power of toons.