$ sudo apt-get update
$ sudo apt-get install gnat-4.9 

gnat-4.9 is installed in your system.

Make ensure the gnat-4.9 package were installed using the commands given below,

$ sudo dpkg-query -l | grep gnat-4.9 *

You will get with gnat-4.9 package name, version, architecture and description in a table

gnat-4.9 package basic information: