Fill a sink or bucket large enough to hold the dress with cold water. Submerge the gray dress completely in the cold water and soak for 20 minutes.
Add several drops of gentle laundry detergent to the brown stain and rub the fabric against itself. Let the dress soak for another 30 minutes in the cold water.
Remove the dress and launder as you normally would. Remove the dress from the washing machine and examine the stain. Continue if the brown discoloration remains.
Lay the stained garment on top of a white towel, keeping the discoloration facing up. Saturate a sponge or cloth in dry-cleaning solvent and blot the stain until it is wet with the solvent.
Wait for 20 minutes before adding several drops of gentle laundry detergent to the area. Rub the detergent into the fabric with your fingertips. Hold the dress under cold running water to flush the detergent out of the fabric. Continue if the stain is still visible.
Apply several drops of a presoak enzyme treatment to the brown stain. Leave the treatment on the dress overnight before laundering the dress as you normally would and air-dry.