GNAT Community was just a version of the GNAT toolchain/IDE provided by AdaCore. GNAT is still open source and still updated as part of GCC, it is now just recommended to install it using the Alire package manager. Builds of GNAT are also provided on some distros since it is part of GCC.

Alire is just a package and toolchain manager that AdaCore wrote in the style of Cargo. It still runs GNAT under the hood.

I don't think AdaCore was involved in the creation of alire:

https://zaguan.unizar.es/record/79726/files/texto_completo.p...

AdaCore does directly contribute to GNAT which is important.

GNAT was chosen for alire since it is free software: "This work presents a working prototype tailored to the Ada compiler available to open source enthusiasts, GNAT."

Other Ada implementations typically use their own build systems which are different from GNAT's and so they would probably need changes to work with alire.

Alire isn't written by adacore, it's an independent project though adacore has donated to them.

At this point it is becoming the de facto method of acquiring the toolchain and building Ada projects.