gnus-gmane-group-download-format

gnus-gmane-group-download-format is a variable defined in `gnus-group.el'.
Its value is

"http://download.gmane.org/%s/%s/%s"

Documentation:
URL for downloading mbox files.
It must contain three "%s". They correspond to the group, the
minimal and maximal article numbers, respectively.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.