Additionally, you can create a rule to convert your custom PG identifier into a link for the Book Details pane, see:
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I had been using tags for filtering PG titles, never minded creating a custom id (identifier, oops) for those.
identifiers:gut:true will now filter those, as I've just set up this.
(also, I have a "grouped" search which is just a search alias for identifiers. This way I can do just i:gut:true)