You have to stay still as well, this is tragically because of a bug that I couldn't figure out to fix where every time you move horizontally it briefly registers you as not being on the floor.

Godot's Area2D has a way to create custom gravity fields, but it didn't actually have enough to meet my needs (it could only attract towards a point), so I implemented my own gravity fields which could return a gravity direction given a position, created a gravity tracker which kept track of what fields I was in (1px wide Area2D), and then return the final gravity direction. Then it was a simple matter of setting the up direction to be the inverse of the gravity direction, and using dot products to get lateral movement working properly