How can I attach a microphone and speaker to the Arduino GSM shield?
"Differential layout" means that the PCB tracks should be a close, a constant distance apart and close to equal length. This has the effect that most electronmagnetic interference applies to both lines equally, and will vanish when one is subtracted from the other.
TVS is to protect against electrostatic discharge; you may get multiple kilovolt spikes when a human touches the device. The TVS diode shorts this to ground to protect the GSM module.
Yes, it's a jack for a headphone-and-microphone accessory.