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.
We don't know every place where gangs store their weaponry (aside from on themselves, which is more than enough to support my point), otherwise they'd be assed out of fire power right about now.
Also remember the mindsets of panic-laden civilians compared to sociopaths and bloodthirsty criminals; the latter is gonna adapt quicker to an apocalypse and loot/obtain everything they'd need while they've got the chance. Particularly since gangs are gonna know where they kept their weapons; if they're still in tact, gangs will be the first to get 'em obviously.
If you recall your history lessons, humanity has the unstoppable urge to rebuild itself after disasters. Take Hiroshima for example: After being nuked, crime did not take over. No, criminals fled to find better places to survive.
Is Hiroshima a poor gang-infested place today? No it is not.
But, didnīt Hiroshima had an actual goverment support to itīs rebuild?, too much diference with the apocalipsis scenario you are arguing, I also believe gangs would be in charge, but thereīs so many variables.
There are variables that support your theory of civilians being able to hold their own, Japan has proved that time and again with all the disasters going on. At the same time, Japan isn't exactly known for it's gang violence and powerful drug circles. Anywhere that supplies and life necessities are more scarce than normal (countries with high poverty and thus, crime rates before and after a nuclear apocalypse), you won't see as much unity, and instead more chaos and "finders keepers".
Think about it: The world. At war.
Supplies are low, everyone is saving food and materials. Countries unite and divide. Weapons are being held in top priority as well.
Then there are gangs. Tell me, did we ever worry about gang violence during this turbulent time?
The closest example to something doesn't necessarily mean it's a good example to argue with. :P
I'll say it again: an important city falling under nuclear destruction isn't comparable to the entire world falling under nuclear destruction.