Using RealSpeed, if you right-click on the speed graph, one of the items is "altitude" -> you should then see the altitude data.

However, you will find it is either zero (ie: disabled in the firmware) or varying wildly due to the altitude only being accurate to a few hundred feet.

Also, the GT31 only samples data at 1Hz - you will probably find that your "flight" only lasted a second or so... ie: not long enough to sensibly measure.