My memory is failing so I'll need to ask for help. I want to ADD a GFCI device into an existing single gang box in the kitchen to provide additional outlets.. Box contains a single pole switch for the overhead light. There are three (3) sets of wires in the box. All three WHITE wires are wire nut'd together, the three black are separate. How do I proceed?

Many thanks in advance.

Howard

If this is in the kitchen you cannot use this feed for a receptacle.
You'd be better off running a new line to a new box and not adding to the existing one.
You can also bring a feed from another receptacle in the kitchen.

Wow... that is NOT good news. We have already installed a new solid surface countertop AND backsplash and I don't see anyway to run a new line to the box or from the receptacle at the other end. Now I have to figure out what to do with the larger 2-gang hole already in the backsplash. Yikes.

You can find a two gang plate for one toggle switch on one side and blank on the other side, see photo attached.

The issue is kitchen counter receptacles are to be a dedicated appliance circuit, and cannot be on lighting circuits, and vice versa.

If you need a receptacle there, and I am sure you do, the only solution is to have a new 20 amp #12 cable installed to this two gang box to power the new GFI outlet.

Can the other switch work the disposal?
You could install an X10 switch with the other switch, filling the hole and allow control of any load.
"We have already installed a new solid surface countertop AND backsplash and I don't see anyway to run a new line to the box or from the receptacle at the other end."
That's 1 reason I don't do/like romex, Many times it is Obsolete the moment it is installed.
I may use if the customer insist, still don't like.
Romex is safe, allowed by code.