If we are going to be technical about it, when there is only swelling of the bottom surface's of the feet, that can be referred to as Pododermatisis, when there is swelling along with some type of ulceration and core that can be called Bumblefoot, therefore, this duck appears to have Pododermatitis. Articular Gout is pretty uncommon with ducks, so it's quite unlikely, but still a possibility nonetheless.
In case's where the bottom of the feet is merely intact, a vet may preside an NSAID and, in addition, an antibiotic such as Enrofloxcin, Cephalexin, or Ciprofloaxcin to name a few. It's also important to correct the underlying cause, or try to better prevent it from happening again.