From what I have read the GSM.h and the GSM.cpp files have to be modified because they are for an earlier version 1.00 of Arduino,
the lines inside these files to be changed are
#include "WProgram.h"
#include "NewSoftSerial.h"
Wprogram.h is now Arduino.h
and NewSoftSerial.h is now SoftwareSerial.h because they are now included in ver 1.01 of Arduino
#include "Arduino.h"
#include "SoftwareSerial.h"
any reference to NewSoftSerial must be changed wherever it is used,
however I am still having problems myself after these changes with errors on uploading.