Hi.. Can understand your concern.. As per your query it seems you have a draining sinus opening and the smelly fluid is pus. It may occur due to either a deeply decayed tooth due to which there is infection reaching below the tips of root causing formation of pus in that area and the condition is called Periapical abscess which may be draining out through an opening on the surface of gums. Other reason can be infection of gums which may either be confined to gums and the condition is called as a Gingival abscess and if the infection involves the underlying bone also it is called Periodontal abscess. If decayed tooth is the culprit then treatment of the tooth by performing a root canal treatment or extraction( depending on the condition of the tooth) can be done along with proper antibiotics and painkiller will cure your problem. If it is due to gums then deep scaling followed by antibiotics can heal the sinus.. Only listerine gargles are not sufficient. You should consult a dentist and thorough clinical evaluation and appropriate x rays will decide the exact nature of your problem and treatment can be done accordingly.. Hope your query is solved.. Thanks and regards.. Dr.Honey Nandwani Arora.