GNAT is the GNU Ada compiler.  It is now part of GCC, and you can
download it through http://ftp.gnu.org/gnu/gcc or any mirror listed at
http://www.gnu.org/prep/ftp.html.

For more info on GCC and GNAT, see http://gcc.gnu.org.