I guess what you mean is a way of recording the GPS data output from the GPS chip built into the phone.
I looked into this a while back and from memory there were a few apps that could record
some of the data, mainly the 'trackpoint' or location data.
AFAIK, there are no apps that can record the equivalent of the Sirf binary data we use to get dopper derived speed and the error correction data.
If the I-phone has a sirf3 GPS chip, it might be possible, but many other brands of GPS chip can not output this data.
Even if you could record this data, you would have a very expensive device that was at very high risk. It would also probably not have the display data that the GT-31 Genie gives you about your previous runs etc.
Better, and less financially risky, to just get a GT-31 and a Paqua bag.