Free standing planter fence
8' long on one leg, 10' long on the other, both 6' tall, resting "on top of the gravel", not anchored into the ground?  The walls will act like giant sails in the wind with the ends leaning over or flapping in the breeze until a strong enough wind breaks the fence.  You should dig the holes and sink 4x4's as you would regular fence posts, cementing them in for support.