Dogs get gingivitis just like people do. They have normal wear and tear and often have tooth problems as they get older. Unfortunately, some dogs are just more suspectible than others are, even though it sounds like your dog had some nice, chewy things that one would have thought would have helped the situation. Perhaps it did and they might have been even worse, had you not given some of the things that stimulated the gums etc.
I highly recommend a product called Biotene Drinking Water Additive, esp if your dog hates having teeth brushed. The Biotene Drinking Water Additive is completely safe and effective way of reducing tartar, plaque and gingivitis, as it has natural enzymes in it derivived from milk. These enzymes have anti-bacterial, anti-fungal and anti-viral properties and help protect your dog's teeth and mouth. One of my dogs had stained teeth, that were starting to look really bad. We started using this product in his bowl and within a month, I could tell a huge difference in the way his teeth and gums looked. The stains gradually disappeared and gums were much more healthy looking.
They also make an oral antiseptic maintenance and treatment gels. These are great products!! I've used them with my own dogs with much success. You can find these products at many vet's offices or by ordering from the many online vet suppliers or Amazon.com. Look on Amazon at all the positive reviews. Here's a couple of links...
Dog owner for a life time. I also train and show my dogs.