There shouldn't be any issue with giving them vitamin C, just watch the dosage. I'd go with Calcium ascorbate powder, not Ascorbic acid, gentler on the stomach.

Dry food is probably the cause of the dental issues.

Here is a link that might be useful: Natural Nutrition for Dogs and Cats

Our little pug developed mast cell tumors about 8 years ago. The specialist we took him to at a university veterinary school said there was anecdotal support for Vitamin C delaying the growth of the tumors. For the next five years, Pugsly (about 15 lbs.) took at at least 2000 mgs. of vitamin C a day. Ultimately, he redeveloped the cancer, but I will always believe the vitamin C delayed the metastasizing.

I'm not recommending that dosage for every dog, but I still give our other little pug at least 500 mg. of Vitamin C daily, just for general health.

If you give your dog too much vitamin C, they will "throw it off," i.e., eliminate it through diarrhea/loose bowels and/or vomiting. You can then back off/reduce the dosage.

I buy gel capsules of time-released vitamin C (no additives) at a health food store. They are more expensive, but they are easy for the pugs to swallow. I wrap the capsule in a piece of soft cheese. They never turned it down!

I need to correct my last post--they're not "time-released." I checked the bottle, and they're just vegetable-gel caps from Whole Foods.

Vitamin C is an antioxident so a good general supplement for dogs, so certainly no harm. Ester-C is supposedly more absorbable, but I've never seen documented scientific proof of that. That said, I give my dogs Ester-C :) You would dose to bowel tolerance. Start with 500mg per day.

Best treatment for gingivitis is a good dental cleaning by your vet, followed up with daily home care.