Hi Nima,
No bother at all
The first versions of GPO that used the Kontakt player(s) did work directly with Composer, as they were a detectable MIDI device. The Aria player, however, needs the virtual MIDI cable. Fortunately after you get the virtual cable installed, it thereafter runs just as if it were "direct". The main thing you'll need to remember is to run GPO BEFORE running Composer. Kind of like making sure there's water in the pot before you set it to boil
Do let us know if you have any more questions - there are other GPO users around as well.
ttfn,
Sherry