We finally have GNAT FSF 13 builds available in Alire, in a separate branch for now. To try it, add an index like so:
alr index "--add=git+https://github.com/alire-project/alire-index#gnat-fsf-13.1.0" --name=gnat_fsf_13 --before=community
Apologies for the delay, this release was particularly difficult to build for no good reasons (GitHub CI, time available, etc.). Also note that the compilers hosted on macOS x86_64 are missing, I will try to make them available ASAP.