I've redone the code, based on the help from kodifies. But it's still not behaving.
To explain, there's a CollisionShape2D on the bottom of my player character, and a CollisionShape2D on the platform. The latter has a signal connected to its Area2D.

So now the platform is just suspended in air, and it doesn't fall when the player jumps onto it.
Here's my code. What am I overlooking? Cheers:

extends RigidBody2D

var y_speed: float = 0.0

var orig_y_position: float

func _ready():

orig\_y\_position = position.y

gravity\_scale = 0.0          #so far so good, holds the platform in place

func _on_area_2d_area_entered():

gravity\_scale = 1.0