The gnat's do serious damage only in huge invasions and they rarely happen. The fungus gnats themselves can make tasty treats for sundews, and they seem irresistibly attracted to them, so they're easy meals too. I have lots in my house seeking some cool air after a large surge in temps here in PHX. For the larvae, let them be unless it's a big swarm. They're found in basically every soil there is and don't do much harm. Also, if you have terrestrial bladderworts, you should introduce them to their soil. They'll get sucked up by the traps and will feed the plant. If you do have particular damage though, just take the plant out of the pot, gently rinse the roots with some mineral-free water, and introduce the plant to new media.