Can I expect a perfectly flat chest after gynecomastia surgery even though I have a muscular chest and my bodyfat is around 20%?

You bring up a very important issue:  what is the perfect gynecomastia result? Is it a flat chest? Is it a chest without any fat? With some fat?  What is it that comprises the most dramatic, attractive chest for a male?  The answer is that it depends on each patient's own body. In my practice, many of my gynecomastia patients have spent a great deal of time in the gym before coming in, as they have tried diet and exercise. But with the issue of gynecomastia, no amount of dieting and exercising produce the desired result. Taking out too much fat would leave a patient with unnatural contour issues, and taking out too little fat or breast tissue would leave you without the chiseled look you desire. In addition, I perform my surgery without making any incision on the nipple/areolar complex, so you do not need to sacrifice the embarrassment of gynecomastia for the embarrassment of a noticeable scar. The truth is, a perfect result can be achieved. The chest wall is comprised of your pectoralis major muscle, with some amount of subcutaneous fat under it. The breast is an appendage that hangs off the pectoralis major muscle, composed of breast gland and fat, the proportion of which varies with age. The younger one is, the more breast tissue there is. The older one gets, the more fatty tissue occupies the breast space. The reason an outstanding result can be achieved from a tiny incision (4mm) in the hair-bearing skin of your armpit is because specially designed gynecomastia cannulas are used to remove both the breast tissue and the fatty tissue, leaving behind an appropriately thin layer of subcutaneous fat under the skin without placing a visible scar around the nipple. The strokes are counted and the volume from each side, composed of breast tissue and fat, is easily seen and accounted for to achieve a balanced result. With this unique approach, a flat and handsome chest can be produced. 

A flat chest or muscular chest or proportionate chest?  Achieving a flat chest may not necessarily be desirable. As you have stated, you are at a 20% body fat where as any average male should be closer to 10% body fat. Achieving a totally flat chest may result in a disproportionate appearance. It may be preferable to leave some fat behind in order to achieve a normal contour change to your abdomen and outer chest wall. Achieving a flat chest may ultimately produce a sunken appearance if you Do you not have sufficient or adequate muscle bulk beneath it. Your parents will also depend on your skin laxity. If access breast tissue is removed in an individual with loose skin this may result in hanging skin and may not achieve your desired goals. It is best to discuss this with the surgeon of your choice.

Thank you for sharing your question. Unfortunately without photographs or an in-person examination it is difficult to offer you advice on what kind of results you can anticipate.  See a series of surgeons in consultation to discuss in order to get the best advice.