A good mission to farm Gunkers is the mission Getting the band back together.
As your character is helping Wendy tow her boat, your character will shout Gunkers! Immediately 2 Gunkers will spawn, one on your left and one on your right. Upon killing those two Gunkers, another 2 Gunkers will immediately appear on your left and right from the alleyways.
Once all 4 Gunkers have been killed, stand still and let the OD kill you. You'll be right back at your character shouting Gunkers and you can rinse and repeat until the challenge unlocks.
Here's a brief video to help.