Rinse the gesso spot under running water as soon as you notice it. If it's very fresh, this may be all it takes to clean it.
Apply a small amount of pumice soap to any remaining gesso spot and gently rub it with a clean sponge. If it's a delicate fabric, test an inconspicuous area first. Rinse thoroughly.
Soak the spot in undiluted white vinegar if the spot persists. Depending on how set the gesso is, it may take a few minutes or it may take an hour or more. As the gesso dissolves, rub the spot lightly with an old toothbrush, then rinse.