There is nnir.el but it has a few problems.  So I'm starting a
rewrite.  I have something which does more than a no-op, but it's
nowhere complete, yet.  But as the saying goes, release early, release
often...

What are the problems with nnir?

* It includes code from people who may not have submitted the
  paperwork, so it can never be part of Gnus.  I think it would be
  useful to have searching be part of Gnus.

* It has obfuscated configuration that's prone to bugs, too.

What are the aims for nnsearch?

* Make it easy to set up.

* Allow for searching multiple search engines and multiple select
  methods in one go.  For example, you might index your nnml stuff
  with Namazu but also want to use IMAP searching for nnimap groups.

What does nnsearch currently do?

* It can use Namazu to query your nnml backend, if it is named
  "nnml:".

How to set it up?

* Let's say you have this server spec:
      (nnml "" ...other.parms...)
  Change it to read like so:
      (nnml ""
       (nnsearch-root-directory "~/Mail")
       (nnsearch-namazu-index-directory "~/.namazu")
       ...other.parms...)

* Index your mail with Namazu 2.0.x:

    mkdir ~/.namazu
    cd ~/.namazu
    mknmz -h ~/Mail

* Load nnsearch.el.

* Invoke M-x gnus-group-search RET from the group buffer.
-- 
Two cafe au lait please, but without milk.