$ sudo apt-get update
$ sudo apt-get install 389-dsgw 

389-dsgw is installed in your system.

Make ensure the 389-dsgw package were installed using the commands given below,

$ sudo dpkg-query -l | grep 389-dsgw *

You will get with 389-dsgw package name, version, architecture and description in a table

389-dsgw package basic information: