Well, GP or pg is actually a measure of resources invested,.not combat power. At best it is a rough approximation of combat power. And it is even less meaningful in fleet where a chunk of it represents the third and fourth reinforcements which are often not used.
What really matters is the power of your three starting fighters and your first two reinforcements and their ability / faction synergies e.g. Vader's target locks giving Biggs protection or the Falcon calling other Rebels to assist.
I regularly beat fleets with as much as 25% more GP than mine while leveling up.