Seems like your
_physics_process doesn't run at all, cause your player should fall.
You probably wrote something wrong, make sure your
_physics_process looks like this:
velocity.y += GRAVITY
velocity = move_and_slide(velocity, Vector2.UP, true)
Do not multiply the velocity by
delta, since the
move_and_slide function already does, as you can read here.
You should use the formatting feature when posting code here, so it's easier to help you.