We have analyzed the effects of gas on the disk-halo evolution in barred galaxies using N-body/SPH simulations. Our main focus has been the angular momentum (J)~redistribution mediated by the stellar bar and the subsequent evolution of the galactic disk. We find that the stellar bars in disks with 0-8 percent gas fraction are primarily affected by the growth of a central mass concentration owing to the bar-driven gas inflow to the center. The almost abrupt weakening of the bars observed in all models is triggered either by vertical buckling (in gas-poor disks)~or by resonance heating (in gas-rich disks). The dark matter halo serves as the angular momentum sink for the stellar bar, thus compensating it for the angular momentum received from the gas.