I'm a little bit confused for stacking shield on top of Arduino. My question is how to solve the problem that the two shield using same digital pin. Arduino GSM Shield (http://arduino.cc/en/Main/ArduinoGSMShield) use pin 7 for Modem Reset while Adafruit Ultimate GPS Shield (https://learn.adafruit.com/adafruit-ultimate-gps-logger-shield?view=all) use pin 7 and 8 for soft serial connection.
Wont work easily - see if the adafruit library can't be changed to use a different pin. If it can, then you will have to wire that pin in manually to the shield (bend the offending pin outwards so it does not go into the connector of the board below it and the plug in a jumper wire to the pin you changed to to in the library)