As a Roblox developer, it is currently impossible to get the grid/step number associated with the rotate tool under the snap to grid section under the model tab of the ribbon bar.
Currently the only API that I have found that is able to provide anything remotely close to this is the plugin.GridSize which provides the number for the move tool. I think this would be beneficial for any developers that want to integrate scripts/plugins into studio while maintaining some degree of seamlessness.