Grapefruitseed extract works by killing excess bacteria you have, so if the cause for your BO / bad breath is excess bacteria, it will help kill them. However killing them is just a "patch remedy". You have to look at WHY you have excess bacteria in the first place. If you dont treat at the root of the problem, they'll just come back again after a while.
Reasons *why* you may have bacterial overgrowth could include a low functioning immune system that needs boosting, or although it sounds a little unlinked, it may be because the bad bacteria in your gut are really high and you need to start working from there, on your digestion. The 4 R protocol could be of help.