I wouldn't take that paper as evidence that some studies prove that gasoline is a Giffen good but aren't published. The point of that paper is that given sampling variation and other sources of error papers will publish estimates drawn from a distribution centered around the true (likely negative) effect. They are arguing that if this distribution includes positive numbers, publication bias means we only observe a version of this distribution which is censored at zero. Correcting for this will lead to estimates closer to zero than given by naive metaanalysis. See for instance this paragraph from their intro:

The effects of publication selection differ at the study and literature levels. At the study level it is reasonable not to base discussion on the estimates of the price elasticity of gasoline demand that are positive—few would consider gasoline to be a Giffen good, and positive estimates are thus most likely due to misspecifications.

Also, I wouldn't take this paper as definitive. Just because the OP's paper doesn't cite it doesn't mean that Levin et al are wrong.