I was trying to do this as well for a top to bottom background gradient, here's a workaround I came up with.
- Create an Area2D with a TextureRect as a child
- Then rotate the Area2D 90 degrees and move it along the x axis the width of the screen.
- On the TextureRect I added my gradient and set the transform > size x and y to the height(x) and width(y) of the screen.
I will probably also set these values in the _ready function of my background script in case I ever decide to change the screen dimensions.