First, you should try and drop some B.F. Higher body fat usually corresponds with a higher estrogen environment. It's also anecdotally reported that losing weight helps reduce the mass of the gyno...shrinking it. Then there is pseudo-gyno that is usually B.F. growth around the chest, or those insecure about some natural imperfections. May I ask how you were, or who diagnosed you?
Now the best studies out there are on raloxifene. This is a pharmaceutical and not a supplement though. There are raloxefine gyno protocols out there that you can look up.
Invictus as pointed out by Chef could be worked into a protocol, or even run solo. It's the strongest legal A.I. on the market.
SNS inhibit-P may help if there are any prolactin issues. This seems more common in those who use specific anabolic, it would still be wise to get a full blood panel.
So gyno is tricky. Every case is different. Different hormones, different severity of mass size, how new or old the gyno is, etc. Hopefully you plan to see a specialist to maybe come up with a protocol, even if you dont plan to use any pharmaceuticals, atleast to check for any underlying issues.