It depends upon the sensor module you are using. If gas sensor output is digital ,you can use digital pins or analog pins to interface.Search for the pin which are used to take digital input and analog input. You can use pins like P0 or P1.
For GSM: GSM uses Tx, Rx, Vcc and Common Grnd for communication. You can use P0.0 and P0.1 OR P0.8 and P0.9 of LPC2148 for serial communication.
Please go through the tutorial:
http://www.engineersgarage.com/arm-projects/introduction-to-arm-microcontroller-lpc2148
http://www.engineersgarage.com/arm-projects/uart-programming-for-lpc2148
hope this helps.