Before you write your response to the presented argument, you'll need to take a stance, and this is what will give your essay direction and purpose. Moreover, it really does not matter if you agree or disagree with the presented text. Thus be sure that the focus of your submission is not whether you agree or disagree with the presented argument. Instead, in the argument essay you want to logically argue in favor or in opposition to the presented topic based on whether it is well or poorly reasoned.

Deciding if you agree or disagree with the conclusion(s) of the argument topic will enable you to organize your thoughts. Regardless of whether you decide to wholly agree or disagree with the provided statements, you'll want to structure your essay in the following manner:

1. In the first paragraph, introduce the argument. This is where you demonstrate that you have considered the what, where, and why of the provided statement.
2. Somewhere in the first paragraph, and most likely at the end of the first paragraph, you'll want to explicitly state whether the presented argument is valid. If you think that the argument is invalid, then simply state that fact. Likewise, if you wholly agree with the argument and you think that it is logically valid, then say so.
3. In each of the successive paragraphs, you'll want to dissect a few (at most) of the reasons that are given in favor of the argument, and you'll need to either explicitly raise objections to each statement or further support each statement. You want to demonstrate explicitly WHY each statement or conclusion that is part of the argument is valid or not valid. In doing so, you are demonstrating your skill at looking at details and seeing how specific details help refute or support a conclusion. That is precisely the skill that you need to demonstrate to the readers of the GRE essay.
4. Just as in the issue essay, you'll want to provide a concluding paragraph where you again rephrase some of the ideas in your introductory paragraph and where you once again state that the arguments that you were given are either valid or flawed. You may also want to reiterate why you think the arguments are or are not flawed.

For the argument essay, remember that the readers are not so much looking for specific details or counter-arguments that you can raise, nor whether you agree or disagree with the argument. Most importantly, do not morph the argument essay inso an issue one. Do not write your stance of the argument -- that is not the goal of the argument essay.but instead just want to see that you can dissect the argument and that you can intelligently respond to any claims that are made. Because of this, the structure of the essay is very important, and, as the above list suggests, you'll want to logically provide a framework and then select key segments of the argument that you think make the provided statement well or poorly reasoned.