gnus-use-long-file-name

gnus-use-long-file-name is a variable defined in `gnus.el'.
Its value is

t

Documentation:
Non-nil means that the default name of a file to save articles in is the group name.
If it's nil, the directory form of the group name is used instead.

If this variable is a list, and the list contains the element
`not-score', long file names will not be used for score files; if it
contains the element `not-save', long file names will not be used for
saving; and if it contains the element `not-kill', long file names
will not be used for kill files.

Note that the default for this variable varies according to what system
type you're using. On `usg-unix-v' this variable defaults to nil while
on all other systems it defaults to t.

You can customize this variable.