When exactly does this happen?
I'm not sure Pannonia is very defensible even with a Roman-Gepid alliance. After all, in our own history the Gepids were allies of the Romans on and off, and as far as I remember I don't think they really put up much of a fight over Sirmium when the Romans decided to retake it. All in all, the Gepids seem like pretty weak partners. I expect the Romans would think, and rightly so, they could do better with a Lombard ally in the region.
I think the problem is that Sirmium in the dark ages is going to invariably lose importance for the Romans as other, more critical threats rear their heads in Asia and elsewhere. The Avars are going to be able to take Pannonia if they want it, I expect. You'd need more than a Gepid alliance to make defending Sirmium a priority, I think.