Installation on Fedora
On the download page you'll find packages for different versions of Fedora. You need to download at least Gnat Config and one of Gnat/GCC and Gnat/GPL. You can install both compilers if you like, or just one of them. You'll probably also want some of the libraries. Be sure to pick those library packages that match the compiler you've chosen.
Install the packages with
rpm --install <package1>.rpm <package2>.rpm ... or
yum localinstall <package1>.rpm <package2>.rpm .... (Yum is more convenient because it resolves dependencies for you.) If you already have older versions of the packages installed, use
rpm --freshen or
yum localupdate. See the RPM and Yum manuals for details.
Also read the page on Gnat Config, which describes the installation of the configuration files and how to do a relocated installation.