You're focused that gangs have power through drug trade. No, they have power through weapons. Yes, they won't be selling drugs in an apocalypse because there'd obviously be no demand for them, but that's got nothin' to do with my point. You know that every drug circle has a shitton of weapons, yes?
When chaos and death is abundant, weapons are going to be power. There won't be a government that'l be able to stop a few thousand people with guns in hand anymore. I believe power is all you'd need for a real apocalyptic situation to get what you want, and the power to end many people's lives from a distance via firepower certainly qualifies as power, particularly when gangs have guns en masse. There won't be civilian groups with enough firepower to stand up to them either; except Texas.
If that still somehow escapes you, in the most simplified statement possible; drug money doesn't matter when you already have a million weapons.