gnus-mark-article-hook

gnus-mark-article-hook is a variable defined in `gnus-sum.el'.
Its value is

(gnus-summary-mark-read-and-unread-as-read)

• This variable may be risky if used as a file-local variable.

Documentation:
*A hook called when an article is selected for the first time.
The hook is intended to mark an article as read (or unread)
automatically when it is selected.

You can customize this variable.