GnuPG 2.1.x changed the way the gpg-agent works, and that new approach no
longer requires (or even supports) the "start everything as a child of the
agent" scheme we've implemented in NixOS for older versions.
To configure the gpg-agent for your X session, add the following code to
~/.xsession or some other appropriate place that's sourced at start-up:
gpg-connect-agent /bye
GPG_TTY=$(tty)
export GPG_TTY
If you want to use gpg-agent for SSH, too, also add the settings
unset SSH_AGENT_PID
export SSH_AUTH_SOCK="${HOME}/.gnupg/S.gpg-agent.ssh"
and make sure that
enable-ssh-support
is included in your ~/.gnupg/gpg-agent.conf.
The gpg-agent(1) man page has more details about this subject, i.e. in the
"EXAMPLES" section.