gnus-auto-select-first

gnus-auto-select-first is a variable defined in `gnus-sum.el'.
Its value is

t

Documentation:
If non-nil, select an article on group entry.
An article is selected automatically when entering a group
e.g. with SPC, or via `gnus-summary-next-page' or
`gnus-summary-catchup-and-goto-next-group'.

Which article is selected is controlled by the variable
`gnus-auto-select-subject'.

If you want to prevent automatic selection of articles in some
newsgroups, set the variable to nil in `gnus-select-group-hook'.

You can customize this variable.