A prescription for sampling isobaric generalized ensembles with molecular dynamics is presented and applied to the generalized replica exchange method (gREM), which was designed to simulate first-order phase transitions. The properties of the isobaric gREM ensemble are discussed, and a study is presented for the liquid vapor equilibrium of the guest molecules given for gas hydrate formation with the mW water model. Phase diagrams, critical parameters, and a law of corresponding states are obtained.