lightly misting leaves won't cause fungus gnats. letting the tops of your pots stay wet for more than 24 hours at a time will be an open door for them.
misting a plant doesn't really do much for it, though. there is no mechanism for a plant to absorb water through the tops of its leaves, some few nutrients will penetrate on contact, but the water will bead up and run off, or evaporate and leave a small stain from whatever was dissolved in it. it does raise the humidity in the area, which will make the plant not need to transpire as much, so it will grow slower, since it's not drawing any nutrients up through it's root system with the water it doesn't need, since you raised the rh