It's subjective, case by case basis. I have seen dostinex + letrozole help with tren gyno that's already developed, but I have seen cases where it didn't help. It's not a guarantee, but it's definitely a better path then surgery - which by the way can leave you with problems.
It might work after gyno onset but as was posted, it will depend on what caused it. The best thing to do is use an AI and drop your doses for a while along with it if you don't know what caused it.