LPG sensor with GSM

hello,Im a newbie in using arduino, Im making LPG leaks detector with GSM, I have my working codes in sensor but I dont know how to put codes for GSM within this codes. Just ignore the codes in keypad.I just put it in so I can use it in when GSM is already working. Please help me with this.This is my codes in my sensor

#include <LiquidCrystal.h>
#include “MQ135.h”
#include “pitches.h”

LiquidCrystal lcd(12, 11, 6, 5, 4, 3);

//keypad
#include <Keypad.h>

const byte ROWS = 4; //four rows
const byte COLS = 3; //four columns
//define the cymbols on the buttons of the keypads
char hexaKeys[ROWS][COLS] = {
{‘1’,‘2’,‘3’},
{‘4’,‘5’,‘6’},
{‘7’,‘8’,‘9’},
{’*’,‘0’,’#’}
};
byte rowPins[ROWS] = {22,23,24,25}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {27,28,29}; //connect to the column pinouts of the keypad

//initialize an instance of class NewKeypad
Keypad keypad = Keypad( makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS);

//Gas sensor
MQ135 gasSensor = MQ135(0);
#define lpg_sensor 0

//buzzer
#define buzzer A3

//LED
int greenPin = A1;
int redPin = A2;

//Button
int button = 13;
int buttonState = 0;

void setup() {

pinMode(greenPin, OUTPUT);
pinMode(redPin, OUTPUT);
pinMode (button, INPUT);
pinMode(lpg_sensor, INPUT);
pinMode(buzzer, OUTPUT);
pinMode(2, OUTPUT);

lcd.begin(16, 2);
lcd.print(“LPG Gas Detector”);
lcd.setCursor(0,1);
delay(2000);

}

void loop()
{

//keypad char
char kpd = keypad.getKey();

//declared gas sensor
float ppm = gasSensor.getPPM();

buttonState = digitalRead(button);

// check if the pushbutton is pressed.
// if it is, the buttonState is HIGH:
if (buttonState == 1){
//Greetings

// turn LED on:
digitalWrite(greenPin, HIGH);
delay(200);
digitalWrite(greenPin, LOW);

if(ppm > 100)
{

tone (buzzer, HIGH);
lcd.clear();
lcd.print(“LPG Gas Leakage”);
lcd.setCursor(0, 1);
lcd.print(" Alert ");
delay(400);
digitalWrite(redPin,HIGH);
delay(100);
digitalWrite(greenPin,HIGH);
delay(100);
}

else
{
noTone(buzzer);
lcd.clear();
lcd.print(" No LPG Gas “);
lcd.setCursor(0,1);
lcd.print(” Leakage ");
lcd.print(ppm);
delay(1000);
digitalWrite(redPin,LOW);
digitalWrite(greenPin,LOW);

}
}

if(buttonState == 0)
{
lcd.noDisplay();
noTone(buzzer);
digitalWrite(redPin,LOW);
digitalWrite(greenPin,LOW);

}

}

#include <LiquidCrystal.h>
#include “MQ135.h”
#include “pitches.h”

LiquidCrystal lcd(12, 11, 6, 5, 4, 3);

//keypad
#include <Keypad.h>

const byte ROWS = 4; //four rows
const byte COLS = 3; //four columns
//define the cymbols on the buttons of the keypads
char hexaKeys[ROWS][COLS] = {
{‘1’,‘2’,‘3’},
{‘4’,‘5’,‘6’},
{‘7’,‘8’,‘9’},
{’*’,‘0’,’#’}
};
byte rowPins[ROWS] = {22,23,24,25}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {27,28,29}; //connect to the column pinouts of the keypad

//initialize an instance of class NewKeypad
Keypad keypad = Keypad( makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS);

//Gas sensor
MQ135 gasSensor = MQ135(0);
#define lpg_sensor 0

//buzzer
#define buzzer A3

//LED
int greenPin = A1;
int redPin = A2;

//Button
int button = 13;
int buttonState = 0;

void setup() {

pinMode(greenPin, OUTPUT);
pinMode(redPin, OUTPUT);
pinMode (button, INPUT);
pinMode(lpg_sensor, INPUT);
pinMode(buzzer, OUTPUT);
pinMode(2, OUTPUT);

lcd.begin(16, 2);
lcd.print(“LPG Gas Detector”);
lcd.setCursor(0,1);
delay(2000);

}

void loop()
{

//keypad char
char kpd = keypad.getKey();

//declared gas sensor
float ppm = gasSensor.getPPM();

buttonState = digitalRead(button);

// check if the pushbutton is pressed.
// if it is, the buttonState is HIGH:
if (buttonState == 1){
//Greetings

// turn LED on:
digitalWrite(greenPin, HIGH);
delay(200);
digitalWrite(greenPin, LOW);

if(ppm > 100)
{

tone (buzzer, HIGH);
lcd.clear();
lcd.print(“LPG Gas Leakage”);
lcd.setCursor(0, 1);
lcd.print(" Alert ");
delay(400);
digitalWrite(redPin,HIGH);
delay(100);
digitalWrite(greenPin,HIGH);
delay(100);
}

else
{
noTone(buzzer);
lcd.clear();
lcd.print(" No LPG Gas “);
lcd.setCursor(0,1);
lcd.print(” Leakage ");
lcd.print(ppm);
delay(1000);
digitalWrite(redPin,LOW);
digitalWrite(greenPin,LOW);

}
}

if(buttonState == 0)
{
lcd.noDisplay();
noTone(buzzer);
digitalWrite(redPin,LOW);
digitalWrite(greenPin,LOW);

}

}

Can you post the code in code tags </> please ?
Can you post all the three files separately ?
Can you provide a link where you found this code ?