Gizzard shad have extremely fast growth rates, it is possible IMO...

Gshad will directly compete with your smaller BG for food. They will also begin to take up a large percentage of your pond's biomass if there are not adequate numbers of LMB and/or other predators to remove large numbers of young. You can expect gshad to be 6" by the end of the first growing season and even larger up to 9" or more. This can put them out of range for most predators in a pond in a hurry. Are you sure they are established in your pond or could this one be a fluke? IME, gshad in ponds of your size is a VERY bad thing. So bad, it may require a complete start over with poisoning.