There’s a really easy way to clean the gunk and slime off the rubber flange of your disposal.  It takes about five minutes and no special equipment, just an old toothbrush.  Maybe some rubber gloves if you’re particularly squeamish.
Half-fill a small bowl with warm water and add a squirt of dishwashing liquid.  Dip the toothbrush into the suds and scrub the top of the rubber flange.  Don’t forget the sides of the thing.  Then (here comes the icky part) lift the edges of the rubber and brush underneath.  This is where the bad smells come from.  Lift each section and brush under it.
That’s it.  You are done and your disposal looks new again.