Source: changelog 2004 entry. Code comment from that changlog commit: 'E-mail posting with a note that gpg interaction requires gpg version 1.2.4 or better.'
I'm not really that familiar with that LiveJournal feature though so I can't suggest much in the way of wording. I'm not sure whether the user's key needs to have been generated using a GPG version of 1.24 or higher, or if it means something slightly different? So, some techie input from someone familiar with posting to LJ using GPG-signed e-mail is probably needed. :o)