Focused ion beam (FIB), assisted, gas etching has several advantages over physical sputtering in many FIB applications. Advantages include etch rate enhancements of 1-2 orders - of magnitude, dramatically reduced redeposition of etched material on sidewalls in high-aspect ratio structures, and reduced implantation of the primary ion species in the sample. Applications which benefit from FIB gas etching include photomask and x-ray mask defect repair, integrated circuit modification for failure analysis, and sample preparation for scanning electron microscope and transmission electron microscope analysis. In this article, a simple model is presented which takes into account the ion beam and scanning parameters, gas flux, and basic material constants. Approximate formulas are given in terms of these parameters and compared to experimental results.