The "gap" is an assumption, an unspoken premise, something that an argument is taking for granted or failing to consider. Often times, the gap/assumption is the conflation of two seemingly similar, but nevertheless distinct ideas.
For example, the boy lowered the costs of his newspaper delivery business. Therefore, he must be making more profit.
Here, the gap is that the argument is treating low cost and high profit as if they are the same thing. They can be, especially if revenue stays the same or increases, but they might not. So filling that gap could be a statement like this: "In the newspaper delivery business, when you lower your costs, you always end up with higher profit."
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For example, the boy lowered the costs of his newspaper delivery business. Therefore, he must be making more profit.
Here, the gap is that the argument is treating low cost and high profit as if they are the same thing. They can be, especially if revenue stays the same or increases, but they might not. So filling that gap could be a statement like this: "In the newspaper delivery business, when you lower your costs, you always end up with higher profit."
Hope this helps