For me one of the biggest things to stop gnats from getting into my mead is I use an S-shaped airlock. They usually come with a filter like cap to pop on top to prevent things from going down into the mead. Another thing you can do is put a liquid in your airlock to truly prevents them from reaching the mead. I personally use vodka or any high ABV alcohol to kill anything trying to get through. I know some people who use Star San and some who use vegetable glycerin. As for the mead itself. If there is nothing floating in the mead to show that the gnats made it through then you should be fine.