Either the GTVA wins completely, the UEF wins completely, or they form a treaty near the end
Or it comes to a bloody stalemate after which neither side is willing to continue the war, but without any signed peace treaty and the chance of the war re-erupting at a moments notice.
Besides a total victory of the UEF is extremely unlikely.
Getting the GTVA to retreat from Sol? Yes. Unlikely but possible.
Completely defeating the whole GTVA? No way. They can barely hold their own against the Terrans. If they go on the offensive, they'll have to fight the Vasudans too, the two parts of the GTVA are just too intervined for the Vasudans to just stand by if the UEF goes on an offensive in GTVA space. And from the backstory (and forumposts) we learned that the Vasudan military is even stronger than the GTVAs, because of their healthier economy and because they didn't pour massive amounts of ressources into a portal-project.
Something that always bothered me about the GTVA was, that they didn't even give the UEF a chance to prove their assumptions wrong. For fifty years they lived in a isolated system, completely save. The shivans must have become little more than stories to frighten children. Now that the node is open again, the thread of another Shivan attack is much closer and far more real. That alone could easily have been enough to turn the UEF into something the GTVA would see as capable and willing to stand against the Shivans. Now add in information about the seconds Shivan incursion and the GTVA might have gained a willing ally instead of an enemy.
While the diplomatic approach might not seem as likely to succeed as the quick overwhelming military solution the GTVA planned on, it does have it's advantages for sure. For one thing even a quick military victory would have cost lives and material on both sides. And for another a war will inadvertadly create a rift between the factions. Or did they really think that the defeated UEF people will just say "thanks for conquering us", even if there is another Shivan incursion? If the GTVA really wins by subduing the UEF I wouldn't be surprised to see a lot of humans join up with the Hammer of Light (both the political and the radical military wings).
Apart form that the war just came at too convenient a time to keep the GTVA high-ups to fully buy that they really only do this for the good of Humanity.