A high efficiency nitrogen rf plasma source has been used to grow GaNAs by diluting the N-2 gas with Ar. This source (an EPI UniBulb(TM) source) was originally designed for use in the growth of pure nitrides at high growth rates. For growth of As-rich GaNAs, high concentrations of active nitrogen lead to the growth of GaN instead of a random alloy. In this work we demonstrate that a dilute N-2/Ar mixture leads to GaNAs films where the amount of nitrogen incorporation varies directly with the percentage of NL in the gas mixture. Films with high structural quality were grown, thus validating the use of this approach.