Hi Gucho,
will the GSP you are looking at chase things?
As others have said you will hedge your bets getting a pup or dog from proven field lines. Yes, there are dogs that don't have any desire to retrieve. One of my pet peeves is breeders who sell dogs into a hunting home with the hopes it will turn into a hunting dog when they have no knowledge themselves on what to look for in a hunting puppy.
With an older dog as long as the dog will chase things you will likely be able to train him to retrieve ducks for you. It may take a lot of work and effort or it may not. It depends on the individual dog, and how much time and resources you have to put into him but that is the same when starting with a puppy too. A natural retrieve would be perfect but if all he has is chase, it's a start. If the dog has no chase then I'd pass on him.
Whether you get the dog or not, come to the seminar so you get a taste of what it's like to train a hunting dog.
Kas