One key problem I see with Gallia being so overpowering. I can easily see them having massive numbers of ships in stock, mass produce them & mothball them in vacuum. However, many of them will be older types having been produced at sometime in the past. This will pose some problems unless they are only partially complete, mainly hulls as everything else would likely be outdated.
Things like drives, computers, life support, shields & weaponry go through advances at times rapidly making them obsolete. Military technology can be bought, stolen & reverse engineered but you only get good at using them by using them in combat. It sounds like Gallia had not really had a competent large-scale opponent for quite some time. Their rebel groups do not really apply. Those would give them quite a bit of experience in small ship actions in the main. You can have good equipment & win tactically but lose strategically if you do not know to use that equipment well enough. A simple example would be the ignorance of the capabilities & usage of our RADAR that could have eliminated the surprise at Pearl Harbor. The proper knowledge & usage could have gotten our aircraft in the air, ships moving & at battlestations as well as having ground forces alerted a hour before the Japanese aircraft actually arrived overhead.
A Navy does not really become competent in large scale fleet actions without actually fighting in them. In history it has been proven that wargames do not really do a good enough job. Look at the problem the US had against the Japanese during the early part of WW2. A Very large percentage of the US fleet were ships dating back to WW1 or just after. The US had a numerical superiority & limited usage of higher technology like RADAR & better SONAR while the Japanese fleet averaged newer ships & some equipment better than ours like the LONGLANCE torpedo & many of their aircraft. Early in the war the US Navy did not make that good of a showing. We thought we were more advanced than the Japanese & we paid dearly for our overconfidence in the early years of the war. You could find similar examples between the British & Germans for the European theater. The Germans had some excellent ships but often fared not that well in fleet actions against the English as the English had more real-world experience in fleet actions.
