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The chemical composition of an extrasolar planet is a fundamental property with little observational signature. Spectroscopic observations of polluted white dwarfs have proven to be a successful pathway to constrain the compositions of disrupted planetesimals but this approach has its own limitations. Here comma we propose a new method to measure the compositions of extrasolar planetesimals by observing the gas debris around polluted white dwarfs. These disks are fresh extrasolar planetary material in a transient stage comma right after tidal disruption and before being completely accreted onto the white dwarf. Recent increases in the sample size of white dwarf gas disks combined with timely advancements in disk modeling efforts have made this new exploration feasible. This proposal will open a noval method to measure chemical compositions of extrasolar planetesimals.