That doesn't really help as GnuPG encrypts the message contents, but doesn't hide sender and recipients. In this case they could still have figured out who sent the messages even if they didn't figure out the contents. Then you apply traffic analysis to proceed to the next step http://en.wikipedia.org/wiki/Traffic_analysis

The only way they could have avoided detection using GnuPG is if they did something like always sending the same length message at the same time every day.