Thank you for asking. How exciting to have a new Gatsby question!
During this time Myrtle is locked away up stairs in the gas station. Her husband suspects (rightfully so) that she is cheating on him and the only thing he can think to do is lock her away until he can move her out of town. It is through this upstairs window that Myrtle is able to see Tom, Nick, and Jordan pull up to get gas.
Myrtle draws two erroneous conclusions from looking out that window, though she has no way (and perhaps no desire) to communicate with the people below. She is actually a little secretive, and it is really only Nick who notices her up there staring down creepily. Unlike the film, in the book she does not smash the window with her hands trying to get Tom's attention.
The first of her wrong conclusions is that the woman in the car, Jordan, is Tom's wife. That isn't true. Jordan is a friend of his, even more so a friend of Daisy's. If anything she could be considered the girlfriend of Nick.
The second of her wrong conclusions is that the car Tom is driving, a big cream colored beast, actually belongs to Tom. In fact, it belongs to Gatsby and Tom just happens to be driving it. This is important because later in the story Myrtle will try to flag down that car, thinking Tom is inside, with disastrous results for everyone involved.
In some ways, it could be said that Myrtle has actually made three wrong conclusions...she has also concluded that Tom actually loves her and that she has some clout to sway Tom away from his wife. I mean, what did she think was going to happen even if she had been able to flag that car down, and Tom had been driving? That he would have thrown his wife out of the passanger seat and let her in?