I think this is a important point, for what I understand from some old posts, there are basically three versions of Gnat:

- FSF: Is GPL but contains an exception that allows you the create programs which are not GPL (basically the same as e.g. g++)

- AdaCore Community : Also GPL, but no exception. Hence your program is also GPL.

- AdaCore Pro: Commercial and allows you to create programs which are not GPL.