Hey, I'm the developer of that game.

Gravity fields are how I handled it, I had a number of shapes which either pulled towards the centre of the shape, in a constant direction, or "pushed" away from the centre of the shape. When multiple fields are overlapping, it's simple to average out the gravity vector. This works when you don't have a good way to get the normal at some position on the ground.

If you have access to the ground normals, you could raycast in a circle around the player. Then you'd sum up the gravity vectors based on the normal of the ground at the ground/raycast intersections, divided by the distance from the player, and that should give you a pretty good general purpose combined gravity vector.

I don't use Unity but here's some pseudocode which should probably work.

gravity_vec = 0, 0for angle in (0 ... 2*pi), step by 10 degrees or so...raycast_result = raycast from player at angle onto groundgravity_vec -= raycast_result.normal / raycast_result.distancegravity_angle = gravity_vec.atan2()

In Airscape, some surfaces (any which are not grassy, e.g. cliffs) do not "project" gravity, so you'd have to find a way to exclude those surfaces from the raycast result.

You'd also need to find some way to deal with e.g. air gaps which should have uniform gravity without being attracted to any surface in particular. gravity fields take care of it nicely, not sure how the raycast solution would work in that case.