From Rosetta Code
GNAT is an implementation of Ada. Other implementations of Ada.
GNAT is part of GNU Compiler Collection. You may find other members of GNU Compiler Collection at Category:GNU Compiler Collection.

GNAT or else GNAT Ada is an Ada front-end to the GNU Compiler Collection. GNAT is a validated Ada compiler. It implements all three consequent Ada standards: Ada 83, Ada 95, Ada 2005, and Ada 2012. The front-end itself is written in Ada. GNAT is available for:

Experimental support is also available for:

GNAT was originally created in the New York University, as a project funded by the government. Presently the principal maintainer of GNAT is AdaCore.

GNAT is freely distributed under GPL, or with exceptions. There is a version downloadable from AdaCore and another one included as part of GNU/Linux distributions. The License Pragma in source code can be used to help detect packages which use GPL instead of a more permissive license.

For commercial closed-source use, AdaCore also offers GNAT Pro and GNAT High Integrity products. These are basically the GNAT supplied with AdaCore support, and licensed with a special exemption. The exemption prevents proliferation of GPL into the proprietary code compiled by GNAT and linked with its run-time libraries.