Velocity is speed with direction which GPS can give you.
If you want reliable and accurate speed then the magnet on the rear wheel and an HE sensor can't be beat. You must have come across bicycle computers that use the same mechanism.
When you get a GPS module you might like to feed its output into u-center on a laptop and take it for a drive in a car around a street circuit. Capture the log file and you can replay the trip in u-center. The trip can be displayed in an x-y plot. You would soon get an idea of how useful it would be on the bike.
If you want to do the same thing on a bike you can connect the GPS to a smart phone and carry in a backpack. Have the phone terminal app capture the log file.