In Godot, it's convenient to export variables so that they can be updated via the Inspector tab. Doing so in C# looks like this:

using Godot;

public class Player : Area2D
{
	[Export] public float Speed = 2f;

	public override void _Ready()
	{}

	public override void _Process(float delta)
	{}
}

Here the Speed variable is exported via the Export attribute that Godot provides. Doing this in F# looks like this:

open Godot

type PlayerFs() =
    inherit Area2D()

    [<Export>]
    let mutable Speed = 2.00f

    override this._Ready() =
        ignore 0

    override this._Process(delta) =
        ignore 0

The same attribute is used, but the key is the mutable keyword. If mutable is not specified, the variable will not even show up in the Inspector tab.