General Relativity

Is it possible for something to have negative gravitational mass so that if i throw it at a black hole it's actually gonna run away from the black hole and not towards the black hole? We don't know if general relativity accurately describes everything in the universe. Quantum gravity allows for other weird forces on antimatter particles like anti-gravity but if you just say we believe in general relativity then it's not possible for antimatter to have anti-gravity well i guess i'm pretty sure einstein is wrong.