First off I love that you are seeking out answers on behalf of your grandfather! We have to take care of family :)
Gum disease can be broken down into two categories: Gingivitis and Periodontitis.
1. Gingivitis is the inflammation of the gum tissue surrounding the teeth. This is normally reversible with treatment and removal of all aggravating debris along with correction of at home care, this normally means more flossing.
2. Periodontitis is when gingivitis (inflammation of the gums) is left untreated for some time and starts to affect and infect the bone and gum tissue surrounding the teeth. This is treatable, however, if left untreated it will lead to tooth loss and has been linked to diabetes and heart disease.
If either of these are caught early the long term prognosis for a patient can be very manageable and maintainable. Most people do not know they have either of these diseases until it is sometimes too late.
And just so you understand the way we diagnosis these diseases is by visual inspection, x-rays and perio chart.
Good luck and thank you for caring for your grandfather.