Hi nakulsasikumar, first don't mix GSM module and GPS module. Tows are different thing (Some module also contains booth tows). GPS device can only receive the GPS data which are send from GPS satellite. Most of the time it follow NMEA protocol. You can find out your co-ordinate by using the GPS device. GSM is booth way communication system. you can use SPRS service, sms service etc through GSM system. SIM300 is a GSM module. it doesn't contain GPS facility. You can make interface with SIM300 through UART. It support AT commands.
I think you can take data from vibration sensor, when no one is stay in the car then if the sensor get ant vibration (That mean some one is trying to unlock the car) then the system send a SMS to the owner of the car. Do you want to make that kind of system?