gnus-summary-thread-gathering-function

gnus-summary-thread-gathering-function is a variable defined in `gnus-sum.el'.
Its value is

gnus-gather-threads-by-subject

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

Documentation:
*Function used for gathering loose threads.
There are two pre-defined functions: `gnus-gather-threads-by-subject',
which only takes Subjects into consideration; and
`gnus-gather-threads-by-references', which compared the References
headers of the articles to find matches.

You can customize this variable.