Kepler isn't known for any work on gravitation. The 'laws' named for him all describe the motions of planets. His descriptions were entirely based on observations, made by somebody else and written down over a period of many years. Kepler just described a structure and motions that would produce those observations ... He never said WHY the planets do what they do.
It was NEWTON, a long time after Kepler, who worked on gravity, and showed that with HIS formula for gravity, planets would HAVE TO do what Kepler said they probably do.