The Myo armband is ready to go with 5 pre-set gestures that use the EMG sensor data. However, developers can combine these pre-set gestures with arm motions (data from the Inertial Measurement Unit (IMU)) to create new gestures. This gives the potential for hundreds of different gestures.
Read about using Myo Scripts to get into creating your own gestures with the IMU.
Developers also have access to raw EMG data from the Myo armband, which can allow them to create their own custom gestures that are completely different from the five default ones.
Take a look at this blog post as a good starting point to learn more about getting access to the raw EMG and IMU data, in order to create your own gestures from scratch.