When the frequency of infrared light and the plasma frequency of highly doped n-GaAs are in resonance (e.g. for a doping concentration N = 7 × 1018cm−3 and a wavelength $\lambda=10.6 \mu$m), the free-electron induced optical nonlinearity is soundly pronounced. At such high doping concentrations it is necessary to extend the rigid quantum mechanical description of the free-electron induced nonlinearity to a multi-valley model. The central valley of GaAs was treated as a fully nonparabolic degenerated electron gas, whereas the satellite valley was modeled as an anisotropic electron gas of arbitrary degeneracy. The following intra- and intervalley absorption mechanisms were taken into account: impurity assisted, thermal and hot polar optical phonon assisted intravalley absorption on one hand and intervalley phonon assisted absorption in equivalent and nonequivalent intervalley absorption on the other hand. The dependence of the different absorption and energy relaxation mechanisms on the doping concentration, free electron heating, optical power density and the equivalent LL-intervalley deformation potential are discussed. We demonstrated for the first time that the behavior of the optical intervalley nonlinearity, i.e. the nonlinear absorption and nonlinear intervalley transfer, strongly depend on the equivalent LL-intervalley deformation potential. In the linear regime the model calculations are in good agreement with experimental results.