The properties of an asymmetric microwave gas discharge that occurs in an argon-filled resonator chamber of a processing plant near the surface of a metal product during the formation on it of a composite structure are experimentally and theoretically investigated. It is shown that with a significant difference between the electrode dimensions, the smaller of which is the product, and the larger are the resonator chamber walls, there is an area of a high energy density of the microwave field near the surface of the product, in which electrons are accelerated to high energies up to hundreds of eV. Diffusing from this area, high-energy electrons are decelerated due to inelastic scattering on the gas molecules that fill the resonator chamber, ionizing them or promoting them to an excited state. We have obtained a differential equation and found its solution which describes the diffusion process accompanied by the loss of energy by high-energy electrons, within the framework of the continuous deceleration model. The estimates of the main physical parameters characterizing the microwave gas discharge plasma are found. The conditions that are most favorable for the energy transfer from the microwave field to the plasma electron subsystem and the product surface treatment are determined.