Adding an outlet to the dishwasher/disposer circuit
This should be fine*

There is not a restriction that a disposal must be on a dedicated circuit unless the manufacturer states that it must be.  Unless you have a high-end beast of a commercial unit, it probably doesn't.

There are rules in the NEC on circuit sharing, and a "fixed" device like a disposal or dishwasher can't use any more than 50% of the circuit capacity if that circuit is to be shared with general purpose lighting/outlets.

What that means in your case is that if the dishwasher shares the circuit, it probably uses more than 50% because of the heater used for drying and water heating.  I think that means you really shouldn't share it with the disposal either and it should be dedicated (so I normally see a dedicated circuit for them).

If the disposal is on it's own or already shared with other kitchen outlets, you are fine to add an outlet.

GFCI required

As you stated, a GFCI outlet is required, and that means you will need a two gang box.  There are switch/outlet combos you could stuff into a single box, but not with GFCI (Link in comments, single gang switch/GFCI does exit), but you probably want more than a singe receptacle.  Upgrade to a two gang (or three!) box and use a GFCI outlet.