first off, your gnats are most likely phorid flies which are common in feeder insect bins. Without knowing what caresheets you are following or about your setup indepth a few things will kill roaches fast

pesticides (are your egg cartons clean?)
mold (mold kills most things quick in confined areas)
high humidity (which can result in mold but humidity alone is enough)
super high temps with no escape also will kill them pretty quick (much faster than cold will)