Evidence That a More Traditional Diet Improves Gum Health
I've written before about how the paleo diet or traditional diets as documented by Weston A. Price can have highly beneficial effects on tooth and periodontal health.
Weston A. Price is of course an old school dentist who travelled around the world in the early 1900's to observe the lifestyles and oral health of a multitude of indigenous populations.
He discovered that those who retained their traditional eating habits seemed to have superb jaw structure and lacked the proliferation of cavities found in more modernized groups. You can read more about his travels and observations in his book, Nutrition and Physical Degeneration.
I recently stumbled upon an article discussing a Swiss reality show "The Lake Dwellers of Pfyn" organized in 2009, in which 10 people lived on a supposed stone age diet. Inadvertently, one of the conditions was that they would not be able to brush their teeth for four weeks due to a lack of hygienic equipment, and researchers at the Universities of Bern and Zürich used this opportunity to study their teeth. 
The participants had no toothbrushes, toothpaste, or floss -- nor did they get any advice on Paleolithic hygiene - but they were allowed to pick or scrape with whatever twigs they could find. As a consequence, the researchers expected their periodontal health to decline.
The thing is, their diet was not really paleo as most people would regard it, since it apparently contained a fair bit of grains, even if of a more ancient type. But, the idea was to replicate conditions that ancient people experienced in that area around 5,700 years ago.
Project organizers provided store houses with grains such as barley and emmer (a type of wheat), wild fruits, nuts, herbs, mushrooms, honey, salt and dried meats. The participants also caught fish and gathered some of their own wild foods. Bow hunting is now illegal in Switzerland, so professional hunters killed animals that the participants prepared.
The Journal of Periodontology published the surprising results in their May 2009 issue (Vol. 80:5, pp. 759-68) 
The subjects' plaque levels did increase, from 0.68 to 1.47 on the Silness-Löe Plaque Index. But actually signs of harm mostly went in the other direction. Bleeding on probing decreased from an average of 34.8% to 12.6%. Probing pocket depth decreased a mean of 0.2 mm (p < 0.001). And, despite the small number of subjects, all these changes were statistically significant (p < 0.001).
Although the subjects developed a lot more plaque, they actually had less gum bleeding and smaller pockets which would signify decreased periodontal disease.
The bacteria population in their mouths changed as well. The overall trend was toward an increase, but not necessarily of the type that cause caries or periodontitis.
Once again we have a connection between diet and the types of bacteria that live inside our bodies. Diet seems to have as much effect on this in our mouth as it does in our colon.
I think this is a pretty compelling, if not limited in scope, advantage of a paleo style diet. Of course, like I said the stuff these people were eating included a fair bit of grains, but I suspect the results probably mirrored some of the observations that Price had of the older more traditional rural Swiss populations when he studied them. They had a higher incidence of cavities compared to far less modernized groups, but on average the rate of caries was less for rural folks than the urban and more modernized Swiss people.