What should I do in the last 5 minutes of a GRE section?

You should still be working on new questions for the last five minutes. Rather than rushing through everything in order to create time for review at the end, it's much more efficient to build in small reviews at each step of each problem so that you can move forward with confidence and know that you don't need to circle back.

Rather than taking the questions in the order that they're given to you, you should solve the problems you're comfortable with first, and then tackle the more challenging ones from most to least approachable.

In the last five minutes, you should be seeing the two hardest problems that you'll actually attempt to solve and doing your best to carefully solve them, and be comfortable with the fact that there may be one or more questions remaining that you're leaving as complete guesses, never having wasted a single minute trying to work through them.