The developers of the mixed-used Gravity project, on West Broad Street in Franklinton, plan to add a residential and spa development called Greenhouse. The project will include a new seven-story building and a renovated one- or two-story building. Together, they will total about 237,000 square feet of commercial and residential space, including 322 apartments. Greenhouse will be built on the 2-acre site of the former A.D. Farrow Harley Davidson dealership at West Broad and McDowell streets, which closed in February.