Gilmer simply asks Heck basic questions. He wants the jury to hear what Mayella said when Heck got to the crime scene. She claims that Tom is the one who had hurt and taken advantage of her. This was already established because these accusations are what led to the trial. Gilmer stops his questions because all the evidence he has is Mayella's word against Tom's.
Atticus asks Heck why a doctor was not called. Heck has no logical answer for this. Heck concludes that it is Mayella's right eye and right side of the face that was injured. She also had finger marks all around her neck. The fact that Mayella's right side was injured will prove to be extremely important. Later, Bob Ewell is questioned and he agrees with Heck's description of Mayella's injuries. Atticus also shows that Bob is left-handed. When Atticus questions Mayella, he asks Tom to stand up. The court sees that Tom's left hand is useless:
His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I could see that it was no use to him. (Chapter 18)
Atticus shows that it is unlikely that Tom could have caused the injuries to Mayella's right side because Tom's left hand is useless. Atticus also shows that it is more likely that the historically abusive Bob Ewell, a left-handed man, is the one who harmed Mayella's right side.