From reading the Debian policy for Ada written by Ludovic Brenta and Stephen Leake: When Ada code is built to produce an executable it is linked with the Ada runtime library. The main difference between the different versions of the GNAT compiler is the license of the Ada runtime library.
- The GNAT Pro Version => GNAT-Modified GPL
- The GNAT Libre Version => GPL version 3
- FSF GNAT Version => GPL version 3 with GCC runtime library exception
GNAT-Modified and GPL version 3 with GCC runtime library exception allow development of commercial and proprietary software. The GNAT libre version is not suitable for commercial software development but excellent for developing free software. The FSF GNAT version seems to be about one year behind the GNAT Libre Version.
I think AdaCore would also like me to point out that with the GNAT Pro Version AdaCore provides support meaning for example that if a bug is found in the compiler one can inform them about this and it will be corrected immediately.