I'll answer that question with a question: why do you think only one developer is working on GnuPG?

There is one full-time developer, but if you look at the git history there were contributions from other people as well:

$ git shortlog gnupg-2.0.9..gnupg-2.1.1|grep :|grep -v ^\ | wc -l 25

If you look at the git summary output you will see that one person Werner Koch was responsible for 82.4% of the work, NIIBE Yakuta was the next closest at 5.4% and it only goes straight off the cliff from there.

To be fair, we switched to git only in December 2011, before that you would need to look at the ChangeLog. BTW, there is a http://gitstats.gnupg.org.

Actually it looks like your entire commit history has been preserved in git, at least starting from 1997:

  $ git log --pretty=oneline | wc -l
      4753

  $ git log ada086d0cec0be47811b3d686017e84e041a5201
  commit ada086d0cec0be47811b3d686017e84e041a5201
  Author: Repo Admin <nobody@gnupg.org>
  Date:   Tue Nov 18 14:06:00 1997 +0000

    New repository initialized by cvs2svn.

The stats from my comment above were just from the parents git log start..end timeframe though.

Here looks to be the stat report from the entire history: https://gist.github.com/anonymous/352d28b459d0fc286dab

I'm sorry, I just accidentally downvoted your comment. Hopefully someone else can cancel it out with an upvote, as HN doesn't let you undo a vote.