You're asking a lot to want her to, not only take new lovers, but to cross into your bizarre world of gangbangs. She either has an interest in black males OR you help her develop that interest. But, it doesn't start with a man's cock. It starts with a general interest in the black male .. physique, mannerisms, likability, etc . Plus, she's just your GF, not your wife, so whether you realize it or not, she's still evaluating you for longer compatibility. You're still competing for her long term interest in you and it can end at the drop of a hat, by you doing or saying something that really rubs her wrong.
Most women prefer their relationship hunts one man at a time; she gets tired of you, she moves on OR she commits to you by marriage OR having your babies.
Have the two of you ever included another man in your sexual activities? Has she ever had sex with another man besides you since the two of you have been dating? Do you know if she's ever been with a black man?
If the answers to these are NO, I'd move slowly or she's going to think you're growing bored or tired of traditional sex with her.
She has no real commitment to you; don't give her a reason to go "hunting".