I recommend using a 50% white vinegar solution, or a 130 degree PBW mixed to the manufacturers recommendations.
Get a medium- to large heat-proof glass jar and squish your gasket in it. Fill the jar half-way with white vinegar and the other half with boiling water, OR PBW mix. Cover tightly with cling-film and let stand overnight, rinse with warm water and dish-washing detergent and air-dry before reinstalling into the fermenter.
Do no use bleach, it can lead to hardening, weakening, and discoloration of the silicone gaskets. That should greatly minimize, if not completely remove, any odor.