We have used density functional theory to investigate the possibility of using graphyne nanotubes for the detection of toxic gases such as formaldehyde and hydrogen sulfide. The effect of these gases on the electronic properties of zigzag nanotubes formed based on alpha-graphyne is studied. The large adsorption distance, small adsorption energy, and small charge transfer indicate physisorption of formaldehyde and hydrogen sulfide on the graphyne nanotubes. It is found a considerable effect on the electronic properties of graphyne nanotubes due to charge transfer from the molecules to the nanotubes. The zigzag alpha-graphyne nanotubes with semiconducting properties become n-type semiconductors with the adsorption of formaldehyde and hydrogen sulfide. Our results show that the alpha-graphyne nanotubes are promising materials for the detection of toxic gases.