I would say that while on a superficial level, men seem to have power over women in The Great Gatsby, there are many instances to support the idea that the women have at least equal power with the men and possibly more, with their beauty or sex appeal, their deceptiveness, and their shrewdness. Let's look at two female characters in the novel, Daisy and Myrtle.
In the marriage of Daisy and Tom, Tom is portrayed as a powerful brute, with "a cruel body," (11). In the very first chapter, we see Daisy reinforcing this impression of Tom, as she shows an injured finger, saying,
You did it Tom.... That's what I get for marrying a brute of a man, a great big hulking physical specimen...(16).
She seems to defer to him when he rants about a book he is reading. We also learn that she tolerates Tom having a mistress. When she speaks of her daughter, she says she hopes that her daughter will
...be a fool - that's the best thing a girl can be in this world, a beautiful little fool (21).
In the scene in which Gatsby declares his love for Daisy in front of Tom, both men try to dominate her, with Tom the victor in the tussle. Thus, there is a case to be made that she is completely dominated by her husband and perhaps somewhat by Gatsby.
On the other hand, there is a strong case to be made that Daisy is manipulating at least two men, Tom and Gatsby, with her beauty and intelligence. She is clearly more intelligent than Tom, intelligent enough to mock him in asides about the racist diatribe he is reading so assiduously. While she appears to be unhappy with Tom's having a mistress, the more we learn about Tom, the more likely it seems that while she does not want to be publicly embarrassed by this, it is relief to her to have someone take Tom off her hands sometimes. As the story goes on, we see that Daisy is not really trapped in this marriage after all, that Gatsby can rescue her, and that she chooses to stay in the marriage because it serves her own needs. She manipulates Gatsby to maintain his adoration and then relies on his chivalry to save herself from a possible manslaughter charge. Who is dominating whom, really, in this story?
We see Myrtle, Tom's mistress, a sensuous woman married to John Wilson, at Tom's beck and call. Tom and Nick stop by the Wilson garage and Tom tells Myrtle,
I want to see you....Get on the next train (30).
Tom and Nick then meet Myrtle in New York, where he keeps an apartment for their affair, and Tom talks Myrtle into having Nick for a visit. We learn during this visit that Tom has apparently told Myrtle he cannot leave his wife because she is Catholic, and Catholics cannot divorce. This, of course, is not true. John, who, when he learns of Myrtle's relationship with Tom, locks her in the house, planning to take her away, certainly dominating behavior.
However, with her sensuous appeal and shrewdness, Myrtle, until her sad ending, has managed to manipulate both Tom and her husband fairly successfully. From Tom, she gets whatever material possessions she desires, clothing, an apartment, and a dog, as well as the opportunity to show off her wealthy and "important" lover. John she completely hoodwinks with her lies, until nearly the end of the story.
While these women do seem to be dominated in some ways, they use what they have to gain power over the men in their lives, too. And at the end of the story, it is clear that it is the women who are responsible in many ways for its outcome. It is Myrtle's sexual sway over Tom and her husband's humiliation over having been cheated upon that lead to her death. And it is Daisy's power over Gatsby that leads to his. In that time and place, certainly, women were generally dominated by men, but as in all times and places, women had their ways.