Are you talking about connecting to your GPS in order to stream GPS data to your iPad or do you want to power your iPad from the GPS cradle? The first is not really possible; the iPad doesn't have a 'regular' USB connection. Yes, it comes with a cable that converts the plug on the bottom of the iPad to a USB-style connection, but it doesn't implement the full (or even partial) USB functionality.
If you want to simply power your iPad from the GPS cradle then that might be possible but, as you probably know, the iPad's charger is a 'special' 10W unit that provides far more current than a 'normal' USB connection. So if you plug your iPad into a normal PC or laptop USB connection you'll find that the iPad displays 'Not Charging' because the current from this USB connection is very low by comparison with the iPad's charger.
I'm sorry if I've misunderstood your question. Don't hesitate to get back to us and I'm sure they'll be someone who can provide a better answer than mine!!
Tim
Scotland