GPS Data Accuracy on Inclines
Next to longitude and latitude data GPS gives also altitude information. It's up to the receiver's software designer to decide if she uses that information. She should, and then the speed is determined by  


  \$ Speed = \dfrac{\sqrt{(\delta \mbox{ } longitude)^2 + (\delta \mbox{ } latitude)^2 + (\delta \mbox{ } altitude)^2}}{\delta \mbox{ } time} \$  


Note that distance as a difference in latitude is constant (111.1 km per degree), while for longitude it's function of latitude: 111.1 km per degree at the equator, 71.4 km per degree at 50 degrees north or south, for instance.  

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The BU-353 supports NMEA's GGA sentence, which gives you essential fix information including longitude, latitude and altitude.