You have more than one independent system. A typical glass panel install will have a primary ADAHRS (air data attitude and heading reference system which contains a MEMS magnetometer, gyro, accelerator, pitot static pressure transducers, and processor) and a secondary ADAHRS both connected to the primary flight displays. Should one fail, you have a fallback for your PFD. As a backup, were you would normally have a vacuum gyro, you have a small glass panel (the garmin offering is the G5) which contains it's own built in ADAHRS and battery and will continue running even with the entire aircraft electrical system disabled.