When using my library the RX and TX for the software serial communication is by default located on pin 2 and 3 respectively. You will need to connect RX (pin 2) to TX on the GSM module and TX (pin 3) to RX on the GSM module.
You would need to connect GND as well as you said.
Also you will need connect the powerpin (pin 4 by default) to the status pin on the GSM module - this allows the Arduino to turn the GSM module on and off.
The pins can be set in the constructor like so: https://github.com/Lauszus/GSMSIM300/blob/master/examples/GSMSIM300/GSMSIM300.ino#L11
, so you can change them if they are already in use.
Hopes this answers your question.