The objective says I can unlock Malachias by killing all the gargoyles. I killed 9 of them and spent the last 10 minutes of my round looking for more and there were none that I could find. I did this on a custom game, does it need to be in the main game quest or am I still missing some gargoyles?

I'm pretty sure you have to be in the main game mission to unlock this character. It worked fine on my end and I've not played any custom missions yet.