Sedimentary rocks in Gale crater record important information about the climatic history and evolution of Mars. Recent gravity measurements and modeling indicate strata encountered by the Curiosity rover have a very low density (1680 ± 180 kg m−3) and thus unusually high porosity. Missing in these models, however, is the role of deeper crustal porosity on the observed gravity signatures. Here we simulate the impact formation of Gale crater and find that impact generated porosity results in a negative gravity anomaly that decreases in magnitude with distance from the basin center. Incorporating this expected post-impact gravity signature into models for the bulk density of strata in lower Mt. Sharp, we find a best-fit density of 2300 ± 130 kg m−3 for an impact into a target with no pre-impact porosity. Models incorporating pre-impact porosity result in densities that are up to 200 kg/m3 lower. These revised densities increase the maximum potential burial depth of rocks along the rover traverse, allowing for the possibility Gale crater may once have been filled with sediment.