We've been dealing with a similar condition with Percy. Our vet diagnosed it as "stomatitis" which is a more severe form of gingivitis, but also has to do with his immune system's reaction (or OVER-reaction) to bacteria in the mouth.
Anyway, Percy got so bad, he couldn't eat crunchy food and his mouth was so sore and red that he couldn't yawn without it hurting a lot! Its a very painful condition and came on very quickly. He stopped eating and within 2 days was thin as a rail, so off to the vet we went.
He got a cortisone shot - to tame the immune system- and liquid anti-bacterial stuff called Anti-robe. Within 2 days he was eating like a horse and mouth pain was gone. His teeth a still healthy, white and roots are good, but we now have to watch him like a hawk to control the gingivitis (and keep it from becoming the more severe stomatitis). We have Anti-robe on hand and currently give him once-a-day doses for 10 days and then 10 days off. We're trying to avoid having to have another cortisone shot, and so far we are winning, but he will always need some kind of medicine to help control the problem.

So, don't wait to get a vet to look at bad breath mouth and red gums. it can get worse fast and a young cat may not have the stamina to fight off the infection.