In the event that the wave left/right gestures are inverted, your Myo armband may have incorrectly detected which arm you are wearing it on. You will need to take the Myo armband off (the Myo will vibration) and put it back on. Then, perform the sync gesture. If the set up gesture is performed correctly, you will feel a short vibration.
When wearing Myo on your arm, the USB port must face towards your wrist. If it faces your elbow, Myo will think it is on the wrong arm and your gesture may be inverted.
Putting the Myo armband on.