Especially cereal and other grain products, trust me a gnat is nothing.
Just tear that piece off, feed it to the dog(s) and finish the sandwich, problem solved.
You might not want to get a dart frog either cause the fruit flies never all stay in the tank. I have OCD but trust me after 7 years volunteering in a lab that raises frogs, gnats don't bother me near as much, they become a fact of life. One time I think some escaped from another lab cause we had an invasion of flying red eyed Drosophila that were definitely not from our stock.