The results of experimental investigations and computer modeling of the magneto-oscillations of the capacitance of surface layers with a two-dimensional gas in the narrow-gap semiconductor HgCdTe with direct and inverted band structure are compared. The structure of the Landau levels is calculated by a model that has a clear physical interpretation and is based on the reduction of matrix equations to Schrödinger-like equations with an effective potential in which the terms responsible for nonparabolic and spinor-type effects are easily separable. An analytic approach is developed for describing magneto-oscillation phenomena in the two-dimensional gas of materials with a quasirelativistic spectrum, and the emergence is noted of new theoretical parameters (compared to the parameters of materials with parabolic bands). Finally, the parameters of level broadening in spin-orbit split subsubbands are determined and the dominant scattering mechanisms are discussed.