We introduce the Newtonian Gravity Concept Inventory (NGCI), a 26-item multiple-choice instrument to assess introductory general education college astronomy ("Astro 101") student understanding of Newtonian gravity. This paper describes the development of the NGCI through four phases: Planning, Construction, Quantitative Analysis, and Validation. We discuss the evolution of the instrument through three versions, including the refinement of a set of four concept domains and nine examples of items to illustrate how expert review, student interviews, and Classical Test Theory statistics informed our approach. We conclude that the NGCI is a reliable and valid instrument. (Contains 10 tables, 9 figures, and 1 note.)