Disclosed is a multi-nozzle and skimmer assembly for introducing a process gas mixture, or multiple process gases mixtures, in a gas cluster ion beam (GCIB) system, and associated methods of operation to grow, modify, deposit, or dope a layer upon a substrate. The multiple nozzle and skimmer assembly includes at least two nozzles arranged in mutual close proximity to at least partially coalesce the gas cluster beams emitted therefrom into a single gas cluster beam and/or angled to converge each beam toward a single intersecting point to form a set of intersecting gas cluster beams, and to direct the single and/or intersecting gas cluster beam into a gas skimmer.