SerialGSM for GSM shield without SoftwareSerial
SerialGSM is designed to work with software serial, as you can see that from the class declaration in the SerialGSM.h header:

class SerialGSM : public SoftwareSerial {
...
};


Unfortunately, changing it to use hardware serial is more complicated than it looks. There are two potential ways you could go about it:


Derive from HardwareSerial instead of SoftwareSerial. This would give you flexibility, but you'd have to take care of passing all the right data to the HardwareSerial constructor, which is quite messy.
Don't derive from anything. Use the global Serial object, and manually replace all serial function calls with their Serial.xxx() equivalents. This is probably easier, but it restricts you to a specific serial port.


Both approaches involve quite a lot of coding, so they aren't trivial tasks. It's also important to note that SerialGSM currently uses the global Serial object to output debug information. You'd have to remove or modify that before converting to hardware serial, or else it could interfere.