As the 2015 business model of the AdaCore
sell the GNAT Ada compiler licenses, the GNAT
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
As of 2015 alternative technologies might be:mbeddr
with its Jessie plugin
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