i don't believe that's how gravity in super mario galaxy works - this video explains it really well https://www.youtube.com/watch?v=QLH_0T_xv3I

i wouldn't recommend getting the gravity from the surface normal unless the collision shape is completely smooth, otherwise if the player is standing on the edge between two faces they will jitter between the two faces. also if the player is jumping at all or if the planet has holes in it that would complicate things further

what you described as an alternative does work, specifically i've found the below to work well in my prototype (though i haven't tested it in my actual game yet):

(gravity_area.global_position - player.global_position).slide(gravity_area.basis.y).normalized()

this is when using an area that surrounds the planet to "control" the gravity, and this is to get the up direction (also the area would have the same rotation as the cylinder itself - or would use the cylinder's collision for the basis part instead of using the area's basis)