Photocatalytic hydrogen production is a promising and sustainable technology that converts solar energy into hydrogen energy with the assistance of semiconductor photocatalysts. Herein, we investigated the geometric structure and electronic and photocatalytic properties of single-walled GaS nanotubes under the framework of density functional theory with HSE06 as an exchange–correlation function. This paper presents the first study on the geometric structure, electron, and photocatalytic properties of single-walled GaS nanotubes. The results show that the strain energy and formation energy of GaS nanotubes decrease, while the structure is more stable, with increasing radius. Our study shows that after rolling from the slab, the nanotubes undergo a transition from an indirect band gap to a direct band gap and have appropriate band gaps for absorbing visible light. Moreover, it is speculated that the large disparity between the effective mass of electrons and holes can reduce charge carrier recombination. Among them, the nanotube with a diameter larger than (35, 0) showed promising band edge positions for photocatalytic hydrolysis redox potential with pH values between 0 and 7. Based on these properties, we believe that GaS nanotubes will be promising in photocatalytic hydrolysis.