What should I do to communicate with GSM module?
The compiler does not know what PString is. It is not declared in any of the libraries you have included.

You could rewrite to something like this. 

strcat(buffer, "Pintu Dibuka");
n = strlen(buffer);
digitalWrite(pintu,HIGH);
sms.SendSMS(n,buffer);


Please change the other block in the same way. 

There are more errors. You also need to change "int n[20]" to "int n". 

That is a good start.

Cheers!