Don’t think BillboardGUI is able to do that. The best way that I know to get what you wanted is by using Beam with FaceCamera set to true and TextureSpeed set to 0. Here i attached an example to help you:
BeamPointer.rbxm (3.7 KB)
This was also mentioned in some other post : BillboardGuis that don't rotate vertically? - #7 by rbxRunix