Shakespeare
One of the most discouraging things about studying for the GRE is confronting Shakespeare. An exam on Shakespeare alone could keep you studying for some time.
But don't worry. Shakespeare will be on the exam; there's no doubt about it. But of the 230 questions, at most 9 of them will concern Shakespeare, and of those, 4 or 5 will deal with reading it, as opposed to identifying it.
Here's what you do: if your goal is to score high on the GRE, and you want to know the answer to every question, pick five plays that you think have a good shot of appearing on the exam. Strong candidates are King Lear, Hamelt, MacBeth, Henry IV parts I and II, and maybe Othello. For this, read the Spark Notes summaries of the plays and characters, and maybe the significant speeches from those plays. Apart from that, don't waste your time too much.
Another thing to do is to look at famous Shakespeare characters. Below I link to a few likely candidates who, though their plays probably aren't worth studying, their characters are.
And don't forget that of those 9 questions on Shakespeare, probably 3 of them are going to be on sonnets. I list a few likely candidates, but again, even if you don't spot a sonnet as Shakespeare's, you can still get most of the questions by being able to read and interpret it well. Also, Shakepearean sonnets have a recognizable form.