Hi Nick
Welcome to the forum.
Most of us have sailed at some stage and my dad nearly bought one of these back in the 60's, but instead went for a Mirror dinghy to build with me only because at 14 feet this was too big to go on top of his car.
Originally designed by Jack Holt back in the fifties the GP (General Purpose) 14 was designed in a similar fashion to the Mirror i.e. it could be sailed. rowed or have an outboard attached. The Mirror had a thin transom about an inch thick so the outboard could be attached directly, however doesn't the GP 14 have some decking on the transom, so would possibly require an outboard bracket of some kind?
I've sailed these in the past at the IYPC in Iver and there were some there with a hatch cut into the transom decking, however now most GP14's seem to have a transom horse making it impossible to use this method. You could bolt a mounting board to the transom but the bolts would have to be water tight and this could be removed if used only occasionally. Finally a Transom Bracket of some kind would work too, something like these...
http://www.angleseymarinesupplies.com/auxilliary-outboard-brackets.phpSo, where do you sail and do you have any photos?