app-crypt/johntheripper should also build the gpg2john binary: http://www.openwall.com/lists/john-users/2012/08/19/1
GPG support appears to be part of bleeding-jumbo
Correct: equery f app-crypt/johntheripper-jumbo | grep gpg /usr/sbin/gpg2john /usr/share/doc/johntheripper-jumbo-1.9.0_p20211129/README.gpg.bz2 I think we can close it then.