I suspect it will be a problem because: 'The GSM Shield uses digital pins 0 to 9" and the Arduino BT uses the normal hardware serial pins (0 and 1) to talk to the Bluetooth adapter. The features of the GSM Shield that use pins 6, 7, 8 and 9 can be turned off and those pins can be used for other things. The Arduino BT requires pin 7 to reset the Bluetooth adapter.
Rather than using the built-in Bluetooth adapter of the Arduino BT you might want to get something like the BlueSMiRF from SparkFun.com which should work with a software serial port on any two spare Arduino pins.