Ada was sponsored by the U.S. Department of Defense (DoD). During the 1970s, the DoD realized that it had a huge software maintenance problem on its hands. Thus, it started work on a new programming language that met its stringent requirements of code maintainability and reliability. Ada was first standardized by ANSI in 1983 and was later updated in 1995 with the release of the Ada95 standard.
Work on a gcc-based Ada compiler was started at New York University and resulted in gnat, the GNU Ada compiler. Work on gnat continued at Ada Core Technologies Inc. (ACT), which maintained it for some time before it was eventually integrated into the main gcc source tree. Every so often, ACT used to release a GPL copy of its most recent work and made it available, along with some prebuilt binaries, at ftp://cs.nyu.edu/pub/gnat/. Their latest release, gnat 3.14p, required gcc 2.8.1 to build. To be precise, gnat's source was provided with a patch that had to be applied to the gcc sources, and an ada directory that had to be copied into gcc's source directory.
Unfortunately, this led to all sorts of problems. For instance, gcc 2.8.1 was fairly outdated and most gcc versions found in recent distributions failed to build it properly. Hence, if you wanted to use the 3.14p release, you first had to install an old compiler on your system and use it to build gnat. Obviously, this wasn't an endearing prospect.
More recently, ACT's work on gnat has been integrated into the gcc CVS and is now part of gcc 3.2. Though you still need a gnat binary to build the Ada compiler, the integration of gnat into mainstream gcc is likely to simplify the use of Ada in embedded Linux systems in the future.
Apart from the ongoing effort to integrate gnat into mainstream gcc, there are two online projects you may find helpful if you are interested in Ada programming in Linux. First, The Big Online Book of Linux Ada Programming is a collective work started by Ken Burtch with the main goal of providing a complete online reference manual for Ada programming in Linux. The manual is available at http://www.pegasoft.ca/homes/book.html and has a couple of mirrors.
Second, the Ada for GNU/Linux Team (ALT) provides a number of ACT-independent binary packages, RPMs, and patches at http://www.gnuada.org/alt.html. The group also provides a number of links to packages providing Ada interfaces and bindings to popular libraries, such as GTK, XML, and X11.