Bleeding is not something that is associated with acid reflux (unless you have severe erosive esophagitis, and in that case you don't spit up blood, it is passed through your digestive tract). If it was any type of substantial amount of blood you need to see a doctor immediately. If it was just a tiny trace of blood it could have been from your gums or even a minor nosebleed.
Well only you can know for sure if it is your gums. Take a cloth and place it to your gums to see if that is the source of the blood. If not, it could be broken veins in your esophagus. That is if you are having trouble with your GERD and acid is backing up into your esophagus. It kind of depends also on the amount of blood you are spitting up. Sincerely, searchin