First, you haven't indicated how long after breast reduction. It is normal to see small amounts of bleeding on gauze after the surgery but you should consult your plastic surgeon to insure that what you are observing is to be expected and not a complication.
Thank you for the question.
Generally, it is best to run these type of questions by your plastic surgeon (given that it appears you are only recently out of your surgery). He/she is in the best position to advise you and is ultimately responsible for your care.
Based on your description, more than likely you are not experiencing a complication but best to check with your surgeon.