Why the GSM Shield is discontinued? What options are available?
A retired product is just a product that isn't supported anymore. It doesn't mean that this product is not usable anymore. Because the GSM shield is discontinued by Arduino.cc itself, you could buy Arduino.org (link to the GSM shield 2), or even an other model by Adafruit. Here is a link for many options of GSM shields on Adafruits' website.

A last option would be to buy it from a chinese seller on Amazon or Ebay. Though they are cheap, they are usualy very crappy and there are not many libraries for them.