Uncle Mingo does a lot more for Chicken George than just teach him how to gamecock—he becomes the kid's legit father figure.
For the most part, Uncle Mingo remains entirely separate from the other slaves on the Lea farm. He stays with the gamecocks; they stay in the fields. They don't hang out much. It isn't until little George stumbles across Mingo and his chickens that the old man gets another person in his life. Though he has his reservations, he ends up loving George like a son.
It takes a long time for George to realize this. Instead, he's too caught up in a bizarre dance with his biological father and owner Massa Lea in which he desperately seeks the man's approval… only to get squashed repeatedly. In the midst of this, however, Uncle Mingo is there to teach Chicken George about gamecocking, life, and what it means to be a man.
Unfortunately, it's not until Mingo's death that George realizes that the he was "his teacher, his friend, his nearest to a father he ever had known" (98.32).
It's a heart-breaking moment for sure, but also a transformative one—it drives George to realize the importance of freedom and start saving up to buy it for his family. Think of it as Uncle Mingo's final gift to his surrogate son.