Hi Nishan,
I believe Jan Thomas Nygaard is correct regarding licencing. So long as the user is already licensed for the backend system there is not extra cost.
Yes you can download the NW Gateway Add-On from the SAP Service Marketplace (SMP). You will find it here:
I also suggest you take a look at the help and installation guide here: http://help.sap.com/nwgateway
to choose the right deployment option (e.g. Hub or Embedded)
If you are already on NW 7.4 
... then you get it by default and there is no additional install.
Hth,
Simon