Hi Pravin,
Which AVR microcontroller are you planning to use? Most of the microcontrollers have only one serial port and hence to connect two serial devices like SIM300 and GPS you have to use switching techniques on the modules, and as far as I know that is the only complexity in your project.
You can find seperate projects on interfacing SIM300 and GPS with AVR under the section
EG LABS > MICROCONTROLLERS > AVR
Hope you find them helpful.