So there are a couple things to unpack here.
First, fungus gnat larvae live in wet soil and feed primarily on fungus, hence the common name. They're generally not pestiferous, but can become an annoyance in homes when people over water their potted plants and the adults emerge and fly about. However, in the absence of fungus, larvae can feed on healthy plant roots, causing some physical damage but also creating holes in which fungus and other pathogens can enter. This is mostly a problem with newly sprouted and young plants that haven't developed a robust root system.
Fungus gnat adults don't feed on plants, their mouthparts aren't capable of it.
Finally, the fly in the photo is definitely not a fungus gnat. It's much too large and robust. It might be a shore fly, which are occasional pests in greenhouses. Shore flies feed on algae as larvae, which can be present when plants are overwatered or water is left standing. Neither the larvae or adults damage plants and, like fungus gnats, they're mostly just annoying as the adults fly about.
However, that all being said, the photo doesn't have enough detail for me to confidently identify the fly. As the post indicates you are in Indiana, I suggest you contact your local county extension office and submit some flies for identification. The link below contains a list of the county offices: