How to increase my galactic power?

Develop toons. GP was changed a while back, it is literally a measure of how much resources you have put into all your toons.

Increase level, add gesr, put on higher dot and leveled mods, increase abilities, and add stars.

GP(to me) doesn't mean squat. You have solid g12 toons already just focus on important ones. Like you said, 2.7mil with only 3 g12? I'm at 2.1mil and have 14 g12 and they're all important characters

If your main goal is to increase your GP, then focus on your pilots. Developing them will at the same time improve your ships. Best value for your resources.

... or take all your un—geared low level characters to level 53 and equip them with low level gear and increase all their abilities to at least rank 3.

Having a higher GP helps in one area of the game, which is during Territory Battles deployments. Other than that, it’s pretty much irrelevant. To raise your GP, just do as others have suggested: level up all your crap toons to 50th level or so, which is cheapish on credits. Raise all of their abilities to level 3, which doesn’t require any rarish Mk3s. Equip your crap/excess Mods on them, just as a place to store them.