In a theoretical study of gas adsorption on carbon nanotubes (CNTs), we conduct a series of ab initio simulations for a “gas–single-wall carbon nanotube” system for gases of particular practical interest: He and NO. The obtained values of adsorption energy reveal preferable localization sites of absorbed He atoms as well as their dependency on the adsorption distances. A significant effect of NO adsorption on CNT electronic properties is demonstrated. The effect of the presence of a vacancy on the adsorption nature is analyzed. It is shown that under the influence of vacancy formation the CNT structure undergoes reconstruction that enables chemisorption of NO molecules.