If you really want to try it, then buy the Plaque Off Animal (no brewer's yeast) instead of the Plaque Off Cat. The downside to that is that not all cats will eat it which is why they added the brewer's yeast to the cat version. Please also be aware that it is high in iodine and high in phosphorus, so if there's the slightest concern about kidney problems or Hyper-T, then you can't give it at all. It has, in some studies, also been shown to contain organic arsenic.
You could also try giving him L-Lysine which will help if the gingivitis is being caused by some sort of virus. It does seem that gingivitis is difficult to treat in cats and that the only certain way of controlling it is regular dentals and possibly brushing the cat's teeth.
Like Chris, I'm not all that convinced by any of the "products" that claim to help, especially after trying Plaque Off already, but some people do say they've had decent results with it so it's possible it may help for some cats.