Why not just replace the plug in the wall with a GFCI plug? You can get them really cheap at Home Depot or Lowes. It is very easy to switch out just turn the power off to the plug before you work on it. Plug in a lamp to the plug to verify the power is off. Follow the instructions that come with the plug, it is very straight forward.

I recently put in three dedicated 20 amp circuits to my garage and put GFCI plugs on all three. Each plug took about 10 minutes to wire up and test.