All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.

Whoever experiences bleeding gums while fasting must spit until his saliva runs clear — that is, until the bleeding has stopped — and then rinse his mouth to purify it.

If any of the blood enters his internal cavity (jawf) unintentionally, then his fast remains valid. And Allah, the Exalted, knows best.