How to search for a specific message?
There are several ways for this, too. For a posting from a Usenet group the easiest solution is probably to ask groups.google.com, if you found the posting there, tell Google to display the raw message, look for the message-id, and say M-^ the@message.id RET in a summary buffer. There’s a Gnus interface for ‘groups.google.com’ which you can call with G W) in group buffer.
Another idea which works for both mail and news groups is to enter the group where the message you are searching is and use the standard Emacs search C-s, it’s smart enough to look at articles in collapsed threads, too. If you want to search bodies, too try M-s instead. Further on there are the gnus-summary-limit-to-foo functions, which can help you, too.