1) She wanted a baby, Nick didn't
2) Nick is very harsh to her and pushes her down and leaves.
1) He wanted a baby, Amy didn't
2) (This is the question that needs to be answered)
Let's bounce to the end of the movie when Nick gets angry enough to back Amy up to the door, HARD. This indicates that he is capable of hitting Amy. Sure, he was pushed to the limit and he reacts that way. But he is quite capable of hitting Amy.
Everything in Amy's journal is not a lie. The initial portions are real and slowly moves to her falsified version. This part where Nick pushes Amy down is somewhere in between. The argument Amy talks about in her journal is - how she wanted the baby but Nick didn't. The actual argument, if an argument did happen, would have happened the other way around - where Nick wanted a baby but Amy didn't. However, by this time Nick was banging another chick. Their arguments have been discussed to be not as intense, not enough for him to physically attack Amy over the want of a baby. It is shown that Nick has become distant and doesn't really care for Amy much. Besides, Amy needed a strong reason to backup the purchase a gun. She comes up with Nick's violent act to declare that she was scared of Nick, scared enough to buy a gun.
These elements point in the direction that Nick did not push Amy down though he was capable, he never cared enough to argue that intensely.
Here's a link that talks about other big plot-holes in the movie:
Gone Girl's Plot Holes Explained