GSM and XBEE with Arduino Uno

Using Arduino Networking, Protocols, and Devices
6

Looking at the schematic for the GSM shield, it uses pins 0 and 1 or pins 2 and 3 for communication between the Arduino and the GSM shield.

There are at least three different XBee shields. That one is, by far, the worst. It uses the hardware serial pins (0 and 1) to communicate with the Arduino.

So, you could use both shields, with the GSM shield jumpered to use pins 2 and 3, using SoftwareSerial to communicate with the GSM shield, and using Serial to communicate with the XBee.

Forget about trying to debug anything.