Fantastic Neil , I am a fan of GP and sadly missed out on one of these, that was on Ebay last year !
Its a very old tech full calendar, Im not sure on the movement but probably a GP adapted Adolf Schild or Peseux check out the Gp chronicles collector blog if you have not already some great Gp watches .
You can contact GP and they will give you info on the watch or you can get a extract ( I think ) but this has a cost attached ( same as Omega )