We present an orthotropic elastic analysis of frictional granular layers under gravity by studying their stress response to a localized overload at the layer surface for several substrate tilt angles. The distance to the unjamming transition is controlled by the tilt angle α with respect to the critical angle αc. We find that the shear modulus of the system decreases with α, but reaches a finite value as α → αc. We also analyze the vibration modes of the system and show that the soft modes play an increasing, though not crucial, role approaching the transition.