Character Analysis
Father
Gabriel Grimes shows up in the novel first as a mad Daddy, a hard-working preacher who rules his family with an iron fist. The image of Gabriel as someone who rules with a double-edged sword, who provides for his family but who also terrorizes them, shows up in the exchange between Roy and Elizabeth:
"You can talk about your Daddy all you want to," said his mother, picking up her battle with Roy, "but one thing you can't say—you can't say he ain't always done his best to be a father to you and to see to it that you ain't never gone hungry."
[...]
"I just don't want him beating on me all the time," he said at last. I ain't no dog." (1.1.52-55)
So Gabriel fulfills the traditional role of the father who works hard to make sure his family is clothed and fed, but he also has a nasty habit of beating his kids like they're dogs. (Although, who beats dogs? That's just mean.) This duality is a major part of Gabriel's character. He's like Two-Face, without the coin-flip gimmick.
Stepfather
So why is Gabriel such a jerk? Good question. We don't think the novel tells us everything about how he got to be like he is, but we get some pretty good clues. One of the biggest motivations behind his hatred of John, in particular, is the fact that John is not his biological son. But this isn't just your run-of-the-mill evil stepparent thing. This also has to do with God.
Gabriel was a huge sinner—drinking, sexing, the works—before he saw the light and changed his ways. When he finally got around to that light-seeing, he had a dream where God made him a promise:
And now up this mountain, in white robes, singing, the elect came. "Touch them not," the voice said, "my seal is on them." And Gabriel turned and fell on his face, and the voice said again: "So shall thy seed be." (2.2.97)
This means Gabriel's kids will be holy. Cool, right? Gabriel sure thinks so.
So with this hot-off-the-presses dream promise in his pocket, Gabriel asks Deborah to marry him. She does, but, oops, is unable to have children. In a moment of weakness, Gabriel has an affair with a coworker and she ends up pregnant. He's sure this isn't the holy seed God was talking about, since the child is not his wife's, so he sends her away to Chicago, where she dies in childbirth. His son, Royal, never finds out Gabriel is his father and dies in a knife fight. Dang.
Okay, so Gabriel feels guilty and also doesn't have any of these amazing holy kiddos that he was promised when he moves to New York. Right off the bat he meets Elizabeth and her son, John. He thinks he's finally getting a two-for-one: he can pay for his sins with Deborah and Royal by marrying Elizabeth and adopting John, and can also get his holy elect lineage enterprise going.
When he meets little Johnny, he promises Elizabeth that he'll love him like his own flesh and blood:
"He won't never have to fret or worry about nothing; he won't never be cold or hungry as long as I'm alive and I got my two hands to work with. I swear this before my God," he said, "because He done give me back something I thought was lost." (2.3.261)
Sounds like a good deal, and we have to take Gabriel for his word that, at the time, he meant it.
So what changed? Why did he become such a total jerk towards John? Well, Roy came along, and that really clinched the deal. With the birth of Roy, Gabriel's biological son, he doesn't really need John anymore to fulfill the promise from the dream. Gabriel can't see:
[…] how could there not be a difference between the son of a weak, proud woman and some careless boy, and the son that God had promised him, who would carry down the joyful line of his father's name, and who would work until the day of the second coming to bring about His Father's Kingdom? For God had promised him this so many years ago, and he had lived only for this. (2.2.115)
So there's your motivation. The problem is, Roy isn't really into being one of God's elect.
The night that John is saved, Gabriel should be happy. He himself preaches, is a deacon, and considers holiness to be a pretty important thing for his family. So why doesn't he celebrate John's conversion? We'll let him tell us:
Neither of his sons was here tonight, had ever cried on the threshing-floor. One had been dead for nearly fourteen years—dead in a Chicago tavern, a knife kicking in his throat. And the living son, the child, Roy, was headlong already, and hardhearted: he lay at home, silent now, and bitter against his father, a bandage on his forehead. They were not here. Only the son of the bondwoman stood where the rightful heir should stand. (2.2.105)
Royal, Esther's son, and Roy, Elizabeth and Gabriel's son, are both named Royal because Gabriel believes that his heirs will be royalty, according to his vision. But both of them get into violent situations that endanger or end their lives, leaving him, once again, without an heir.
Gabriel calls John the "son of the bondwoman"; not his own son, but hers. A "bondwoman" is an indentured servant or slave, so that shows how he feels about his wife, Elizabeth. He has rejected John and gone back on his offer to love him like his own. Evil stepmothers step aside, because this evil stepdad takes the cake.