Fungus gnats usually appear when the soil stays way too wet. I grow Dracaena marginata: http://garden.org/plants/searc... and let mine dry out quite a bit between waterings; they are very forgiving plants. As long as the roots are not rotted I'm sure your plant will survive; you can cut back those stems and if they are not soft and mushy, just stick them in the soil in the same pot as the mother plant. It can take months and months but eventually the main stem of the mother plant will produce new branches and growth; usually just below the cut area as shown in this photo:
