You can acces the Left Stick direction of joystcik 0 Like that :
var axis = Vector2(Input.get_joy_axis ( 0,JOY_AXIS_0 ), Input.get_joy_axis ( 0,JOY_AXIS_1 ))
axis= (0,1) if at right side
axis=(0,0) if centered
axis=(-1,0) if at left side
axis=(1,1) if at top right corner
etc ...
see : https://docs.godotengine.org/en/3.0/classes/class_input.html?highlight=input#class-input-get-joy-axis
Warning : axis will never be equal to (0,0) or (1,1) or (-1,-1) because of the physical stick that is never really centered or at the end of bounds. so you need tocheck for a minimal diferrence