$ sudo apt-get update
$ sudo apt-get install gnat-6-sjlj 

gnat-6-sjlj is installed in your system.

Make ensure the gnat-6-sjlj package were installed using the commands given below,

$ sudo dpkg-query -l | grep gnat-6-sjlj *

You will get with gnat-6-sjlj package name, version, architecture and description in a table

gnat-6-sjlj package basic information: