The answer is no! While bad breath is a possible sign of gum disease, other common causes crop up as well. For example, tonsil stones can cause bad breath. Tonsil stones are created when decaying food debris collects in the crevices of the tonsils and becomes compacted into what are referred to as "stones." You can have bad breath from tonsil stones without having any gum disease at all.
Post-nasal drip can also cause bad breath, as the discharged mucous provides a rich food for the anaerobic bacteria that cause bad breath, helping them to multiply rapidly. Again, this sort of bad breath isn't by itself a sign of gum disease.
However, the most common cause of bad breath is a lack of adequate oral hygiene. Improper brushing and flossing, or simply too little of it, can leave large amounts of plaque still stuck to and growing on your teeth and gums. If this growth is allowed to continue unchecked, the anaerobic bacteria has the opportunity to burrow down beneath the gum line, where they form unwelcome pockets of bacterial colonies, ones that are devilishly hard to reach and clean without the help of specialized tools.
Anaerobic bacteria give off a smell much like sulfur, and it is exactly that smell that's most closely associated with bad breath. Occasional bad breath may merely be a signal that it's time to brush again, but persistent or chronic bad breath can be an early warning sign of gum disease.
If you have chronic bad breath as well as tender or inflamed gums, there's a good chance that you are experiencing some level of gum disease. It's important to see your dentist as soon as possible to seek treatment, not only for social reasons, but your overall state of health.