Using Rx and Tx pins(0 and 1) on Arduino Uno for GSM
You have a choice.

Always:


GND => GND
+5V => +5V


Then, either:


TX => RX (0)
RX => TX (1) 


and use the hardware UART "Serial", but that then means that uploading new code to your Arduino becomes an arduous task since the usage pins 0 and 1 will interfere with that.  So the other option:


TX => Any pin
RX => Any pin


And then use SoftwareSerial to set up a secondary "bit banged" serial port on the two pins you choose.