Say, if you enter a battle, while the enemy fleet have the upper hand (the power gauge is favoring the opponent), is there any way you can come up ahead? Or if you loose the power gauge at the beginning, its adiós muchachos?

Yes, tactics choice and flotilla distribution.

Not really but it depends. The gause is only estimate of power of your ships based on their modules. A well put together synergizing fleet will have lower power on the gauge but will be superior in actual combat than a fleet with more powerful modules that are nor matched to work together. There are also cases where the balance is terrible like slug flak vs missiles where a massively more powerfull fleet with missiles will deal 0 damage to a much weaker fleet with just a bit of flak defense.

But in general outside of those edge cases if both fleets are properly made then there is no way you can do anything. The tactics and placement barely matter for laser/beam fleets so the one with higher gauge will always win.

I see, thanks!

Also, I notice that, like endless space 1, lasers rule the stars.