In a gas-loaded free electron laser, multiple scattering causes considerable degradation in electron beam quality as the beam propagates through the gas-filled wiggler chamber. To determine the effect on small-signal gain, the conventional gain derivation is revised to include a non-uniform optical mode area, finite electron beam energy spread and finite beam emittance and current density, which both vary with propagation. The results show that gain can result from FEL designs with interaction lengths longer than the conventional maximum given by the criterion of accumulated phase shift equal to π. This makes possible a gas-loaded FEL which utilizes a long wiggler and a asymmetic optical mode to enhance the electron-optical field interaction. This design allows substantial gain at wavelengths down to the ultraviolet.