$ sudo apt-get remove libgnat-5 

Uninstall libgnat-5 including dependent package

If you would like to remove libgnat-5 and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove libgnat-5 

Use Purging libgnat-5

If you use with purge options to libgnat-5 package all the configuration and dependent packages will be removed.

$ sudo apt-get purge libgnat-5 

If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.

$ sudo apt-get purge --auto-remove libgnat-5 

See Also,