I can tell you story of my friend after surgery story his chest was completely flat but after some days later it wasn't the same as it just after the surgery. So he went to the surgeon, but doctor assured him it is only swelling nothing else and it will take time to go.
akclinics.org/gynecomastia/
If you want to gain weight so you should take diet properly not use supplements because when ever you don't take natural diet you can face bit difficult during gaining weight.