This is not for the Maths part of the General GRE. This is for the GRE Subject Test in Maths. Feel free to add or comment.
How do I know the definition of rings or of anything on the GRE given that definitions can vary?
I think trying to relearn an entire undergraduate degree's worth of mathematics purely for this test would be an enormous waste of precious time and energy. You can't make this test your life,you have other priorities. I'd begin talking to graduate students who have been through the test-see what they'd recommend. - Mathemagician1234