For the first time first-principles calculations were performed to get the dependences of strain energy and band gap of achiral nanotubes obtained by rolling up monolayers of gallium (II) sulfide and selenide. The hybrid density functional method (with 13% of the Hartree-Fock exchange) within the CRYSTAL17 computer code was used. The empirical Grimme correction was applied to describe the dispersion interactions between layers accurately. As a result of simulations of nanotubes with different chirality and different diameters, the minimum diameters of the stable single-walled nanotubes were determined, which retain the continuity of the chemical bonds on the outer nanotube surface. It was shown that the strain energy dependence on a diameter obeys a classical law of inverse squares and is the same for «zigzag» and «armchair» nanotubes.