HazardsMind:
Unfortunately I've already used pin 2 for some other purpose.
So change that "other purpose" to another pin and leave pin 2 for the GSM.
You can't change the pin on the GSM shield because it is already hardwired to use pin 2. However, you could reroute the GSM pin to another pin with a wire. Please note this will mean you would need to either unsolder the pin from the GSM shield, which I DO NOT recommend, because it could damage the shield; or bend the pin outward and use a male to female jumper wire to move it to another pin. Only then can you change the GSM_TXPIN in the .cpp file.
Would you know what other pins are commonly reserved by GSM shields and their libraries then? I know 7 is typically used for power buttons and 2, 3 for software serial. Since I have to desolder a header to remove the attachment to pin 2 it'd be nice to have an idea of what pins I shouldn't switch to so I can save myself some trouble.