Chip,
Please confirm that there are 4 colors of wires..
White - neutral
Green - ground
Black - Hot in
Gray - Hot through the switch and to the light.
Currently, the Switch box has no neutral wire, so you can't have an outlet where the switch is now.
To add an outlet you would need to connect a white jumper from the white neutral wire in the junction box down into the switch box.
You would want the new switch to be isolated from the new outlet..
You would want the hot side of the switch, and the hot side of the outlet, connected to the gray wire that is now connected to the black wire (This is your 120 volts input)
You would want the other gray wire connected to the output of the switch.
You would want the new neutral wire connected to the neutral on the new outlet.
If none of this makes sense, then I would recommend that you hire an electrician, or a friend that has a background in electrical work.
Thanks,
Jim R.