ExportingGmane is primarily an archival site, and as such, it's important that it's easy to extract the contents again. If a list admin wants to use a different archival method, or just wants a copy for herself, then there has to be a procedure for extracting the contents of a group.
Now, Gmane is a news server, so it's easy enough to just point a news grabber at a group and just say "go". The main problem with this is that you're not getting a pristine copy of what was appeared on the list. Gmane does protocol conversions between mail and news, and you probably don't want those in a non-news-based archive.
So, a different method is provided, too. And it's scriptable.
Gmane has a "download" interface that returns a Un*x mbox file. To download messages 851 up to (but not including) 855 in the group gmane.discuss, just say
http://download.gmane.org/gmane.discuss/851/855You could, for instance, set up a cron job to fetch the new messages in that group on a monthly basis, but you'd have to keep track of which article numbers you want to get.
This download interface tries to reverse all mail-to-news protocol transformations. It does not decrypt encrypted email addresses at present. Perhaps there could be some special interface for list admins where they could supply a password or the like and get a decrypted archive, if the list is encrypted. But that's not implemented yet.
This interface is a slight CPU and bandwidth hog, so if it's abused, it will be shut down. (List admins will then have to get a user name/password thing going.)