Short of Deus Ex Machina overdrive, all evidence is they're going to win.
I'm not so sure about that.
The GTVA can only continue the war as long as the have the backing of their own citicens. And the longer this war with Sol is going and the more GTVA pilots and ships are lost, the more civilians back home in the colonies will wonder why they are even fighting their brothers and sisters.
Should the public opinion ever swing around the GTVA will have little choice but to pull out and try a diplomatic solution I guess.
Furthermore economically the GTVA was in a really bad situation, with all the funds and ressources pumped into the construction of the portal. And now they are fighting a war and that for several month now.
For all we know the GTVA might be only inches away from complete economic collapse.... or not, we simply don't know.
But considering the situation before the war, I'd guess the GTVA can only win the war fast or not at all, unless they get help form the Vasudans (either economically or even military).