You will need

-the triangular goal
-a physical object (square, circle, etc...)

First, select the triangle and edit it by pressing the E button

that should display this :

This time, do not even care about the gravity lines, just look at the 2 concentric circles.


Okay ? now just do a single click in the center of one of the coloured circles.

nothing happened ? well, you have to do the next click in an object, and one of the circles will move into the center of the object you just clicked.

like this :

Notice that it will be invisible to the player to see the gravity link.
SO : you have to place manually the arrow label on the object
(only if you want the player to be noticed, heh)

BUT : The initial orientation of the gravity won't be determined by the line of the goal.
(frankly, the line of the goal becomes useless)
It depends only from the initial orientation of the linked object (rotate it with the red gizmo they said)

That is all.