gistfile1.el

(defvar gnus-user-format-function-g-prev "" "")
(defun empty-common-prefix (left right)
  "Given `left' '(\"foo\" \"bar\" \"fie\") and `right' '(\"foo\"
    \"bar\" \"fum\"), return '(\"   \" \"   \" \"fum\")."
  (if (and (cdr right)			; always keep the last part of right
    	   (equal (car left) (car right)))
      (cons (make-string (length (car left)) ? )
    	    (empty-common-prefix (cdr left) (cdr right)))
    right))
(defun gnus-user-format-function-g (arg)
  "The full group name, but if it starts with a previously seen
    prefix, empty that prefix."
  (if (equal gnus-user-format-function-g-prev gnus-tmp-group) ; line-format is updated on exiting the summary, making prev equal this
      gnus-tmp-group
    (let* ((prev (split-string-and-unquote gnus-user-format-function-g-prev "\\."))
    	   (this (split-string-and-unquote gnus-tmp-group "\\.")))
      (setq gnus-user-format-function-g-prev gnus-tmp-group)
      (combine-and-quote-strings
       (empty-common-prefix prev this)
       "."))))

(setq gnus-group-line-format "%M%S%p%P%5y:%B%(%ug%)\n")