Unlike Microsoft Word, there is no spell check, word count, and auto-grammar correction on the GRE, which provides a basic Notepad to students for typing their Essays.
GRE Analytical Writing is a test of your critical thinking skills and how you develop and support your argument. Generally speaking, a few spelling mistakes will not have much bearing on your score if the errors don’t affect the clarity of what you are saying. Moreover, typos will have an even less effect on your score because they happen due to haste and not personal incompetence.
However, if you make habitual spelling errors throughout your essay, then yes, your score will suffer.
Grammar, on the other hand, is more important because the correct use of grammar is essential to the structure of your essay. Lousy grammar will cost you points, especially if there are multiple mistakes.
Moreover, GRE doesn’t require a word count on their essays. The most important thing is the content of your essay and not its length. It would help if you had an essay long enough to cogently make your case for the topic at hand and refrain from adding any useless information or opinions that don’t relate to the topic.
Generally, a 500 to 600-word essay is considered the right length. Remember, you only have limited time, so going on needlessly long tirades will not do you any good.