I thought GPO already used some of the samples from GOS, but I could be totally off base. Of course, GOS has many more articulations than GPO, but does not include ensemble samples or solo instruments. GOS is sections only. They compliment one another.
I bought GOS/Lite first and then got GPO a couple of months later. I couldn't do without either.
I usually post "new stuff" and experiments on this page: http://kevintweedy.net/Preview/MusicPreview.htm and identify which components I am using. (Personally, I think it makes sense to have both, since GOS/Lite has more extensive strings -- especially the "Full Strings" group -- that are very useful for varying articulations.)