I want to point out a couple very important quotes:
(Speaking about the Thunderbolts)
They were freed by the Thunderbolt Jolt, who borrowed technology derived from X-51 (aka Machine Man) whose flight capabilities were powered by "cancelling out the gravity equation." Now unaffected by Hall's power, the Thunderbolts escaped and continued to battle, ultimately cancelling Hall's access to his power. When Hall regained his power, gravitational force rushed inward, collapsing inward on himself. He was shunted to another dimension once again.
And:
Graviton was last seen, apparently alive and well, escaping the Raft (a metahuman prison). he fought and bested the New Avengers, but was defeated and almost killed by Iron Man after Hall severely injured Spider-Man.
I want to point out that you are only as good as your last appearance, and it seems to me that once a group or hero understands Gravitron's powers that it makes defeating much, much easier. Clearly, Iron Man has an understanding of Gravitron. Since Gravitron was difficult for this New Avengers team to defeat, until Iron Man came along, it shows he was still very powerful. But, looking at the New Avengers, that first appearance, many of them aren't suited to take out Gravitron. Iron Man is. Tony Stark will know of the same information that the Thunderbolts discovered from Machine Man...Tony Stark will have all of his past encounters as information in defeating Gravitron....and, most importantly, Tony Stark has 24 hours to come up with the means to defeat his opponent, an opponent he's met and defeated before.