Can I apply the Great Weapon Master feat's second benefit (-5 to attack roll, +10 to damage roll) to the bonus-action attack from the first benefit?

Question

Say I trigger the bonus-action attack granted by the first benefit of the Great Weapon Master feat (PHB, p. 167), either by getting a crit or a killing blow. Can I then apply GWM's second benefit (-5 to attack roll, +10 to damage roll) to that bonus-action attack?

As a barbarian at level 5 (Extra Attack) , this would mean that I could use this feature a total of 3 times in a turn — that's some heavy damage.

Answer 1

Yes, you can take -5 to hit/+10 damage on the bonus-action attack.

The Great Weapon Master feat (PHB, p. 167) reads:

[...] you can make one melee weapon attack as a bonus action.

and

Before you make a melee attack with a heavy weapon that you are proficient with, [...]

So whenever you make a melee attack (with a heavy weapon that you are proficient in), including one provided by the feat, you can take the - 5 to hit for the +10 damage. This can be done any number of times - so yes, your barbarian could do it on three attacks (attack, extra attack, bonus-action attack) in a turn.