There is no downside to lower gas prices. lower prices on anything is always a positive.
The downside of lower gas prices is that gas prices are too low (according to some people).
I can tell you as a fact that lower prices on some things are terrible. Specifically, low prices can cause something to be overused and then depleted, or it could have secondary bad effects, or it could impede the development or use of alternatives. I could believe this to be the case for gasoline, and could also believe a lower price would be better -- to be sure I'd have to integrate over the entire economy and resources and present and future technology.
Just to give some examples, the Tragedy of the Commons is an example of something being underpriced, and stupidly expensive monopolies are an example of something overpriced.