On the Guide To MIDI Orchestration page, there is a .pdf document that reviews some actual Orchestral Librays -including GPO.
Paul Gilreath seems to be in direct contact to Gary Garritan.
There he wrote about future articulation add-ons for GPO.
Have there been any, or will there be any, ever?
Can I do my own articulation set/switch for an instrument?
Especially useful would be: Sforzando for brass, advanced string articulations- i.e. Spiccato, Con sordino and Sul ponticello