You are kidding yourself to think that your son's behavior issues have anything to do with him being gifted. I have a 9 yo son who is HG, and he probably is smarter than me. He has never tried to pinch his brother's penis. He has never abused anyone in the family. And he wouldn't dream of being mean and then telling the rest of us we have to put up with it! Why? Because we don't put up with it, and those aren't the values of our family.
I totally agree with all of the advice from Stik. I hope you take it to heart, and that you'll get some meaningful interventions from a parenting expert.
Also, I'm curious about what you do when your son is mean to his brother? And how do you follow up when your son tells you you must accept his bad behavior? My children would consider it a total betrayal if I let one of their siblings hurt them. It sounds like this is an ongoing problem in your house. How does your other son feel about it? What are you doing to protect him from his brother? I would also ask yourself why your son behaves in school and not at home. The answer is probably because rules and consequences are enforced at school. He would never get away with hurting another child and school, so he doesn't try.