A master reset clears user setting details, such as the user profile, stored in what is called non-volatile memory. This memory is easily erased via a master reset and handles only a small portion of information the you have setup in your device.
GPS user data* is not stored in the non-volatile memory on USB mass storage devices. Rather it is stored in the mass storage area of the device in a GPX format. As a result, this data is written to the device’s memory the next time it is powered on. Deleting the data from the mass storage area of your device will remove the data permanently.
To permanently remove the GPS data from your device, refer to FAQ article "How do I delete GPX files from my mass storage device?"
*User data consists of waypoints, routes, tracks, and geocaches