You can do this by Connecting GPS Trans pin to Controller Receive, and GSM Receive Pin to controller Transmit pin.
But in this you will not check the returns from the GSM Modem.
So my suggestion is to use a Digital Multiplexer i had used two mux one for Transmit Line and One for Receive line to interface three UART Devices, of which one is GSM/GPRS Modem, One is GPS and one is 232 Based Wind Sensor.
You you can easily do this,
Just keep the multiplexer on GPS and when you need send the sms change the mux to GSM and then back to GPS