I made a merge request #21 https://git.lysator.liu.se/nettle/nettle/-/merge_requests/21 that adds fat build support for GCM implementation on arm64, the patch also updates the README file to stay on par with the other architectures and use m4 macros in gcm-hash.asm (patch provided by Niels Möller), in addition to add documentation comments.
regards, Mamone