All F9 engines are gimballed. The center engines can gimbal further though, for extra control authority when landing. The many smaller engines approach is use

All F9 engines are gimballed. The center engines can gimbal further though, for extra control authority when landing. The many smaller engines approach is useful for re-using boosters. When coming back in to land, the stage is almost empty, so very light compared to when it launched. The problem then is reducing thrust far enough to be able to do a propulsive landing. Even with one F9 engine lit, and it throttled down, there's still too much thrust to hover, so the timing has to be really good to reach zero velocity at zero altitude.