As of May 2022, AdaCore no longer supports GNAT GPL. This has two consequences for the dev-lang/gnat-gpl ebuild: 1. The <http://libre.adacore.com> site referenced in the ebuild HOMEPAGE variable appears to be deprecated in favor of <https://www.getadanow.com>. 2. As near as I can tell, the proper upstream source code repository is now <https://sourceforge.net/projects/gnuada>; but I'm not 100% positive.
The upstream code repository remains gcc.git. Both links you gave are unofficial binaries.
My bad. Disregard. Closing.
It's not wrong that we need to do _something_ to have a better bootstrap path for Ada (once you have a binary, you can build gcc[ada], and then you have your own binary, and you never need another one off the internet again). But it's not really as simple as just changing HOMEPAGE in there.