Actually, whenever you look, Liberty and Bretonia are kinda alone in that fight. Kusari-Rheinland are the economical backbone for the Gallia right now.
As for the numbers, someone stated in the lore that Bretonia has 30 battleships (in comparison, KNF has 10 of them + 2 from Kusari Exile forces), while Liberty has around 100 battleships. Numbers of 500 battleships is perhaps a bit too bug, but I'd stand around 200-300 battleships of Gallic origin, because Gallia technically speaking is twice-size of any given house (they have around 10 shipyards, if I'm not mistaken - Liberty has only 5, Rheinland 4, Kusari 3+1 and Bretonia has only 2 shipyards).
We also should take into consideration that 800 years in isolation, aside of two civil wars, gave Gallia an upper hand, because unlike Sirius there were no territorial disputes and no major fleet loss - while all houses more or less had been impacted by Nomad War only. However, its nowhere close to Rheinland losing entire fleet during 80 Years War - Kusari lost 99% combat capability during Siege of Honshu in said Nomad War.
Kusari again lost their combat capability (again) because majority of Kusari fleet was in Taus and only two battleships managed to survive - yet they pledged allegiance to the original ruler. I think Liberty knows that Kusari is a Gallic puppet, but it's the GMG that fuels Liberty.
While the Libertonian-Rheinland peace might be advantage for the Allied Forces, it also might mean their huge flaw, since Rheinland and Kusari possess quite strong industry and can casually fund Gallia without being interrupted much - this is one of the reasons why Kusari Civil War is needed for the Liberty and Bretonia, no matter who would spark it first - also being a risk for Liberty-GMG supply line from the "north", unless Ellesmere-Sigma 59 route would be used as silent agreement between both.
