While I haven't researched bitter gourd, some data implies that heating can diminish heat-sensitive molecules but increase the bioavailability of others. For instance, prolonged heat may decrease antioxidant activity but improve digestibility. It depends on the cooking method and time. Steaming retains more bioactive chemicals than boiling or frying. I recommend comparing raw and cooked extracts in vitro and monitoring phytochemicals like charantin and momordicin if you're investigating this.