The gmane indexer was taken from
  http://search.gmane.org/~xapian/
and adapted to Debian list archives.

From the original authors:

Much of the code is Copyright (C) Olly Betts 2004,2005,2006

The indexer is partly based on the we:search indexer, which is licensed under
the GPL and copyright Lars Magne Ingebrigtsen.

Debian adaptations (c) 2007,2008 by Thomas Viehmann and also licensed under
the GNU GPL Version 2 or above.