Most common USB GPS receivers come with a USB connector with a UART-to-USB chip built in. That means the GPS receiver itself provides serial UART output. So they can be used with Arduino by replacing it's USB connector with Rx/Tx and VCC/GND connectors. My practical test has proved that this works. What you need to make sure of is the baudrate of the GPS receiver.
Arduino working with modified GPS receiver
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can you share the sketch you used to read the tx/rx data from the gps receiver? i have one and would like to try this on my mega as a new user!
The code is nothing special, just using TinyGPS.