Fungus Gnats will overwinter in the soil and start to fly around when the soil is warmed back up. It does not matter which company bagged the soil or where the soil was found. The job of the gnat is to help speed up the decomposition of the organic material so that the nutrients can be released back into the 'wild' so I can't fault them for doing what they are supposed to do.
I bought a bag of Fox Farm's Happy Frog last summer to see what was so special about it. I keep the bag outside and will bring in a bucket or so when needed for cuttings and so far no gnats have been flying around.
However, if I do see more than one show up in the grow room or anywhere in the basement I will sprinkle a pinch or two of Mosquito Bits on the large tub of soil mix and maybe a pinch on the soil in any of the pots. The last time I had to open my bottle of Mosquito Bits was last summer before I bought the Happy Frog potting soil.
Yes, the gnats can be annoying especially for some of us with eye problems but to me it is not worth the stress involved to be blaming Fox Farm or any other bagged soil supplier.