III-V semiconductor nanocrystals are an important class of optoelectronic materials. However, the gas-phase synthesis of these materials, especially of the stibnides, has been left relatively unexplored. In this study, we demonstrate the synthesis of free-standing GaSb nanocrystals for the first time, using a novel gas-phase process. We show that when elemental aerosols are used as precursors for Ga and Sb, the elements mix at the nanometer length scale as the aerosols pass through a nonequilibrium plasma reactor. At sufficiently high plasma power, the mixing produces free-standing GaSb nanocrystals, with a small amount of excess Ga segregated at the periphery of the particles. The reaction is initiated by vaporization of elemental aerosols in the plasma despite the low-background temperature. Ion bombardment determines the extent of vaporization of Ga and Sb and thereby controls the ensemble stoichiometry and reaction rates.