NFL/NBA now have a Donald Sterling problem with Gayle Benson

How can you prove that without witnesses, it's basically hearsay.
Not all hearsay is inadmissible. Cops use it all the time in criminal cases. "When I approached the suspect, he told me that he had just shot the victim." This is admissible hearsay because it's a statement by a party in the litigation, plus it's a statement most people wouldn't make under those circumstances. It's classified as a statement against interest by a party. The jury decides whether to believe the statement or not. Same thing in this case. If Gail is a defendant in the lawsuit, her alleged statements can be admissible hearsay.