use 8 bit controller, use relay to switch between GSM and GPS. read gps data and send it through GSM. If you use controller with 2 UARts then you can connect it to indicidual UARTs and use it. Both of them use serial communication to interface..
this project is readily available in 8052.com website
As told earlier, its simple serial communication. there are many threads in this forum, if you search you will get your solution, i have already answered in many threads along with circuit diagram and code. and PCB layout.
Dont get confused with TCP/IP.. It will be used when you need GPRS setup.
Right now your requirements do not need it. so any GSM modem will do the work for you. just dont bother about TCP/IP. download AT commands from net, as i am unable to attach it. try to play with GSM modem in hyperterminal. you will get some idea who it works and how AT commands work.
then Interface GPS to hyperterminal with proper baud rate settings and see if GPS data is coming on the terminal window. once you do this come back for further help..
in case you dont get any output you may post it here.
I recommend you: sim548/508. It contains GSM and GPS in one chip. In my country it isn't cheep but it's unreliable.
UART communication (GPS tx rx and GSM tx rx)