Gibson was in his first full season, and in my opinion, he did nothing to lose the spot. He was basically an average pitcher. His big thing was a lack of Ks, which I'd like to believe will come back a bit, given that he was striking out A LOT more in the minors (his minor league K/9 is 8.05 and he slightly outperformed that in AAA). Given that his age, I think Gibson takes another step forward, and I don't think it would be wise if he were not in the rotation.
Back to the topic at hand, I have to think that May is the guy they want to win this job with Milone being the next on the list. May has 1/3 upside, given the amount of Ks he gets. If he can cut that walk rate, he'll be fine. I think the job will be his as long as he performs reasonably well in ST. I hope that's the case at least b/c I don't want to see Milone there unless someone else is hurt.