It depends a bit on what firmware version you have.
If you have the version with the windsurfing enhancements (with four numbers ending with a 'T') from Tom Chalko's website (www.mtbest.net), provided you have properly formatted the SD card, all you need to do is go the the MISC menu, select SAVE TO CARD, then SAVE ALL and you will have on the card the .SBP log, the .SPD file, which is a text file with you top genie max and 10 sec. speeds and some other file called TRK.BIN which I don't know how to use.

If you have the standard firmware you need to go to DATA LOGGER, then SAVE TO CARD and this will get you the .SBP log file.
It's best to format the SD card in the GT-31 first. It has a FAT16 format. Then proceed as above.