Not on a CC3D. There's no barometer. The GPS altitude can only be used for telemetry. The reason for this is because the CC3D doesn't have the memory or computational horsepower (no floating point processor) to be able to run the EKF attitude estimation algorithm that the Revos do. The Complmentary algorithm that the CC3D uses only factors in the gyros and accelerometers. Besides, without a barometer, you really don't want use a GPS altitude for something as exacting as altitude hold. It's far too inaccurate, even with the best GPS and a full satellite atlas you will maybe get within a few meters. When you are talking altitude hold, a few meters is a whole lot. The barometers on the Revos can detect pressure changes down to 10cm.