Yup, GP falls short even as a measure of resource investment much less actual power.
Problem is there really isn't a straightforward way to quantify the myriad combinations of stats, abilities and synergies that constitute a character's power. Zombie for example would be completely useless in a mixed squad but is invaluable in a Nightsisters squad.
Addressing the worst aspects of GP might help somewhat. For example the fact that simply activating a 1* ship causes the pilot's GP value to double.