I believe the Kingdom of Gallia would fall sooner or later, because of one major factor:
GRN cannot be everywhere.
Now imagine that Gallia owns about 50% of Sirius, now what? Kingdom should really try to ease their new subjects. Else there would be a lot of rebellions, guerilla fights as an addition to already going conflict, perhaps with Rheinland-Liberty combined fleet. Gallia isn't USSR, which was lucky enough to not to have the real second front during WW2 or even a major rebellion in the middle of the country.
Now some history. As you remember, Japan decided to attack USA (Japan got pissed off because of Ribbentrop-Molotov pact, as the sidenote). Mutual attack of Japan and Germany would just destroy USSR in 1941 and even later years, because there wasn't enough forces to fight on two fronts in the same time. Add the matter of distance, too.
Same would happen with Kingdom of Gallia, I believe. It's just impossible to fight and keep territory on two war fronts for a longer period of time.
