gnus-parameter-ham-marks-alist

gnus-parameter-ham-marks-alist is a variable defined in `gnus.el'.
Its value is

((".*"
  ((gnus-del-mark gnus-read-mark gnus-killed-mark gnus-kill-file-mark gnus-low-score-mark))))

Documentation:
*Groups in which to explicitly set the ham marks to some value.

You can customize this variable.