Need rules verification/clarification... Halbert + GWM + PM...

[samcifer]

So my (latest) idea is to get 3 attacks per turn with a Halberd using GWM for the -5/+10 effect. With the Polearm Mastery feat, I can attack again as a bonus action using the back end of the weapon for 1d4 + STR/DEX/CHA mod dmg. Can I use the -5/+10 effect of GWM on this bonus action 1d4 attack as well?

[Expected]

Yes, GWM applies to the PAM bonus action because it is an "attack with a melee weapon with the Heavy property."