Gnome doesn't work at least since I started using NixOS half a year
ago, let's not give wrong impressions to newcomers. Packaging gnome3
is still something on horizon.
That is, you can say
security.pam.services.sshd = { options... };
instead of
security.pam.services = [ { name = "sshd"; options... } ];
making it easier to override PAM settings from other modules.