I think that even if Sirius "wins" as in, the war ends and Gallia pulls out, Gallia still wins. They created an economic boom for themselves, established puppeteer influence in two Sirian houses, destroyed a good amount of western Sirian infrastructure, built infrastructure in Tau-23 uncontested, crippled Bretonia's industrial capacity, ran successful intelligence and psychological warfare campaigns, destroyed multiple fleets from two houses and several nation-states, and literally made Planet Leeds worthless.
They lost a pile of military assets. No home ground was lost, no industrial capacity was lost.
Even if Gallia pulls out, they've still won the war.