Grapefruit seed extract work for bad breath or tea tree oil?

Halitosis. Bad breath can be caused by many different factors, including sinus infections, periodontal problems, smoking, cavities, acid reflux, and your diet. Address all of these possibilities with your dentist and physician, and I'm confident that your problem will at least be minimized, and hopefully eradicated.
Alternative methods. Not something that I would recommend. Good oral hygiene: brushing after meals and snacks, using dental floss properly and the use of a tongue scraper is essential. Also see: http://smilesapartcosmeticdentist.com/fresh-breath-control.htm.