As the 2015 business model of the AdaCore is to
sell the GNAT Ada compiler licenses, the GNAT
imposes GPL to all binaries that are compiled with the GNAT.
The mechanism is that the binaries are linked with GPL licensed stdlib.
They even say it out explicitly in their FAQ(archive).

As of 2015 alternative technologies might be:
mbeddr,Frama-C with its Jessie plugin,
ATS that does not impose GPL by having an exception at its stdlib.
May be the 2015-not-production-ready Irdis might also be of some interest.

Ada 2012 Wikibook.

2014_08_09_Ada.tar.bz2.torrentspacerTargets ~1.8GiB.