Gas loading of a free-electron laser modifies the phase-matching condition while the small-signal gain expression remains the same. This permits a wider parameter space than the vacuum FEL, which is particularly advantageous at shorter wavelength operation. Scattering of the electrons by the gas limits the interaction length, but available gains are still high enough to allow oscillation build-up. For example, a 0.5μm wavelength helical wiggler FEL pumped by a 100 MeV electron beam is restricted to a length of 40 cm with a small signal gain of 361, and reaches saturation in less than a microsecond.