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:

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.

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.