It might not be actual Gyno, just mantits, but if it is, there's nothing at all besides surgery. You can lower your body fat % to reduce the appearance of puffiness, but it's a gland issue and it will need to be surgically removed. This is only reccomend if you have severe gyno..
If you aren't using drugs and have gyno then see your GP and get referred. Depends how serious the case is. I agree with the above, it may not actually be gyno.
If you are using steroids then we can't discuss this as it's against forum rules. However, you should look into how to avoid Gyno if you are using. There are ways this can be avoided. Stop using gear until the problem stabilises.
If you are overweight, you probably need to lose weight to get rid of your gyno if it is gyno If you're on roids, you need to go investigate how to get rid of them yourself If you're neither of these things and you should probably go anyway; you should go to your GP
I actually think it's hard to say. Solid mass under your nipple could be describing something that doesn't at all look like gyno and probably is very unlikely to be gyno. It's a bit depends what you mean