Edit: The reason they work together is the "different creature" means someone other than the primary target of the attack. The second target of Sweeping Attack is irrelevant to GFB, as GFB tells you to make an attack on one creature. The second target of Sweeping Attack is more of a victim of circumstance than a primary target.
See, I don't see it that way -- I see the "different creature" as meaning "creature not damaged by the weapon attack", which would mean that you can do the primary GFB damage against either creature you damaged with the Sweeping Attack, and then deal the secondary GFB damage against a third creature within 5ft.
I don't have the book in front of me right now, so I'll have to confirm this when I do get back to it.