CURLING LEAVES: Curling leaves are a very common issue with Calatheas. If your plant is sitting in too much light it will start to burn and curl inward. Move it farther away from the light. The other, most likely, reason is that it is drying out and in an attempt to conserve water, it is curling up. Water it immediately and you will see it unfurl soon.
BROWNING: Brown leaves and tips are also a common issue. This could be that your water has too many chemicals and minerals in it. Try using distilled water and see if that helps.
GNATS: These damn things amirite? While you can use those fun fly traps for your plants, you need to address the reason they are there. With the moistness that Calatheas require, it can make an ideal home for fruit flies. Allow the top of your soil to dry out and instead water from the bottom for awhile.