I think Jessica is talking about an old work box.
Where are all your wires run? In your attic through the walls?
And also Jessica, just to be on the safe side it is better if he calls an electrican. That way it will be to code, in which he could break if he doesn't do it right. Idealy he would run a 20 amp circuit with a GFCI outlet, but could settle for a 15 amp So what size wires will he run? How far is it away? If his box is 100 feet in the garage, then what? Where does he fasten the cables to the joists and on the wall on down?. Where does he hook the green and white wire in his box? What if there is a short and he touches off a neutral? You know that they main wires coming in are always hot right and they also feed a neutral into that box. GFCI outlets are a pain anyway, if you don't know what they are for and the terminology.
My advice, if you have no experience, don't try it.
I have switches on my wall, figuring that they are hooked up to an outlet, but no deal...and my house was built in 1998. Later I found out they were wired to the ceiling fan ((future)) so you can't always assume anything.
Here is my point exactly. I have an outlet in my garage. I recently moved my older fridge there and plugged it in. ((your kitchen fridge has a dedicated 20 amp circuit...)) This was my bad, but I had no idea it was integrated based off panel labeling...
Later...as I found my box was improperly labeled to begin with. That same outlet is on a circuit with my master bath (above the sink no GFCI) which feeds a second, then both my outdoor outlets in the front and back of the house.
With that. I have a fridge, Christmas lights, my wife blow drying her hair with a 5 -7 amp blow dryer. All run at the same time....and me running a wet vac outside??? FAR over 15 amps to code. BUT...it was built to code, because you can have so many outlet fixtures per circuit. All in all.... What if I plug my chiller in outside? BLAMO!