Can I apply great weapon master’s -5 atk, +10dmg feature to a bonus attack?

Say I trigger the bonus attack granted by GWM’s first feature, (bonus attack upon crit or killing blow). Can I then apply GWM’s second feature (-5 atk, +10 dmg) to that bonus attack?

As a barbarian at level 5 (extra attack) , this would mean that I could use this feature a total of 3 times — that’s some heavy damage.