Rock Sling does standard physical damage. The only sorcery that does strike damage is Cherishing Fingers, a DLC finger sorcery.

Not all gravity sorceries do physical damage. Only Rock Sling and and Blades of Stone do entirely physical damage. Meteorite and Meteorite of Astel do mostly physical damage. The other gravity sorceries deal entirely magic damage (Gravity Well, Collapsing Stars, Gravitational Missile).