Apparently the GT-31 records records the speed over ground - This was posted on the windsurfing forum.
"The important thing is for the Doppler speed (i.e. speed over ground) to be written to the track log so that it is available for subsequent analysis (GPSResults, GpsarPro or RealSpeed).
Few devices save anything other than the postional data and there isn't any supporting data to give any degree of confidence in the readings. That's one of the reasons why the GT-31 is so good for speedsurfing... it's basically a great data logger but also has the speed genie, etc.
I haven't any first hand experience of the GH 505 so I can't say what data it records."