Great Weapon Master Critical Hit Resolution

Question

My apologies if my unfamiliarity with the rules makes this an easy question to answer...

I am considering taking Great Weapon Master, but was curious how this would affect my critical hits. I am currently a level 20 Half-Orc Barbarian, meaning that when I land a crit, I can add a bunch of extra damage dice (I believe I am able to add 3 extra dice from Brutal Critical, and Savage Attacks allows me to add another extra die).

However, how are critical hits affected by Great Weapon Master? If I take the -5 penalty before rolling, and then roll a nat 20, will the critical still hit, AND will I then get to add the extra +10 to damage dealt? Or would the roll become a 15, thereby negating the critical hit altogether?

Great Weapon Master states:

Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack’s damage.

This becomes even more important given the other feature of Great Weapon Master:

On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

So how does Great Weapon Master affect critical hits?

Answer 1

Your critical hits are not affected at all!

If you roll a 20 on an attack roll, meaning the die itself shows a 20, it is always a critical hit, as explained on PHB p. 194:

If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. This is called a critical hit, [...]

The GWM damage bonus is also not affected, because only the damage dice are doubled. You can still add your brutal critical bonus dice, but the +10 on damage does not change, because it is not a die.

Answer 2

See page 194 of the PHB:

If the d20 roll for an attack is 20, the attack hits regardless of any modifiers or the target's AC. In addition, the attack is a critical hit...

Great Weapon Master has no impact on this. You could have a -30 on the attack roll and it wouldn't matter. You still hit and you still crit.

The only time this wouldn't be the case would be if you rolled with Disadvantage and thus would need to take the lower of the 2 rolls. To crit with Disadvantage, both dice rolls would need to be a 20.