You are free to organize and develop your response in any way you think will effectively communicate your evaluation of the argument. Your response may, but need not, incorporate particular writing strategies learned in English composition or writing-intensive college courses. GRE® raters will not be looking for a particular developmental strategy or mode of writing. In fact, when GRE raters are trained, they review hundreds of Argument responses that, although highly diverse in content and form, display similar levels of critical thinking and analytical writing.
For example, raters will see some essays at the 6 score level that begin by briefly summarizing the argument and then explicitly stating and developing the main points of the evaluation. The raters know that a writer can earn a high score by developing several points in an evaluation or by identifying a central feature in the argument and developing that evaluation extensively. You might want to look at the sample Argument responses, particularly those at the 5 and 6 score levels, to see how other writers have successfully developed and organized their responses.
You should make choices about format and organization that you think support and enhance the overall effectiveness of your evaluation. This means using as many or as few paragraphs as you consider appropriate for your response, e.g., create a new paragraph when your discussion shifts to a new point of evaluation. You might want to organize your evaluation around the structure of the argument itself, discussing it line by line. Or you might want to first point out a central questionable assumption and then move on to discuss related weaknesses in the argument's line of reasoning.
Similarly, you might want to use examples to help illustrate an important point in your evaluation or move your discussion forward. However, remember that it is your critical thinking and analytical writing that is being assessed, not your ability to come up with examples. What matters is not the form your response takes, but how insightfully you evaluate the argument and how articulately you communicate your evaluation to academic raters within the context of the task.