gnus-summary-make-false-root

gnus-summary-make-false-root is a variable defined in `gnus-sum.el'.
Its value is

adopt

Documentation:
*nil means that Gnus won't gather loose threads.
If the root of a thread has expired or been read in a previous
session, the information necessary to build a complete thread has been
lost. Instead of having many small sub-threads from this original thread
scattered all over the summary buffer, Gnus can gather them.

If non-nil, Gnus will try to gather all loose sub-threads from an
original thread into one large thread.

If this variable is non-nil, it should be one of `none', `adopt',
`dummy' or `empty'.

If this variable is `none', Gnus will not make a false root, but just
present the sub-threads after another.
If this variable is `dummy', Gnus will create a dummy root that will
have all the sub-threads as children.
If this variable is `adopt', Gnus will make one of the "children"
the parent and mark all the step-children as such.
If this variable is `empty', the "children" are printed with empty
subject fields. (Or rather, they will be printed with a string
given by the `gnus-summary-same-subject' variable.)

You can customize this variable.