In this work, the transmission of microstructures based on gallium nitride with different doping levels in the mid- and far-infrared spectral ranges at T = 300 K was experimentally studied. The transmission of the studied structures in these spectral ranges was modeled using the transfer matrix method. It is shown that the contribution of the lattice, according to the single-phonon resonance model, and the contribution of free electrons, according to the Drude model, to the dielectric constant allows one to satisfactorily describe the optical properties of the studied microstructures up to a quantum energy of 300 meV. The absorption coefficient for CO2 laser radiation (photon energy 117 meV) has been calculated. It has been shown that in gallium nitride absorption on free electrons at a given photon energy can be observed experimentally at an electron concentration exceeding 6·1016 cm–3. The optimal thickness of GaN layers for experimental observation of the absorption modulation of CO2 laser radiation in electric field for different doping levels was determined.