Rotation controls for particles in Form are only available with the Textured particle types. Sprites always face the camera.
You should be able to orient the textured particles toward the center of a spherical Form using two gradients, one to drive Y-rotation and one to drive X-rotation. With the strength set to 360 you should get the proper orientation with a black to white ramp from left to right for the y-rotation and a 25% gray to 75% gray vertical ramp for x-rotation.