In va, where we live, its considered the south, there is no anti gel additive in the fuel.
You find that in the northern states, and out in the west, where it routinely gets much colder in the winter.

Once it gets below freezing, diesel will gel. But, its like water freezing, it is a matter of temps, and time at temp. For instance, a glass of water set out on a cold night freezes much quicker and more thoroughly than say a five gallon bucket full, unless the temps are lower, or for a longer period of time. The fuel itself must get below freezing, so large amounts, or tank types may help slow or prevent it.