If you were tempted to join Gaius' cause at all, he was written well enough to the point where he's not a black antagonist to our white heroism.
While Garleans in general are ruthless racists, those are two things Gaius didn't follow to the letter. Keep in mind that Rhitatyn was the first ever non-Garlean tribunus, as well as that instead of just using the Ultima Weapon to mercilessly crush Eorzea under his heel he gave the city-states the option to surrender. He was also opposed to Project Meteor, though it was more out of concern for the destruction of the land leaving nothing for him to conquer than for Eorzeans. Simply put, Gaius had standards unlike the majority of the other Garleans we've seen, and genuinely believed Eorzea would be better under imperial rule.
That said, he was still villainous because he tried to take away Eorzean freedom by force, and while he did have standards he didn't hesitate when push came to shove.
While Livia probably wasn't the best choice to send on that kind of mission, well... it's war, open or otherwise. Anyone resisting is fair game. We don't exactly have the option to tell all the Garlean soldiers in Castrum and Praetorium to stand down, and they're fighting for convictions they hold just as strongly. That is why war is so tragic, and remember that history is written by the winners.