Is GRE scores required before contacting potential advisors?
You can contact anyone, I think. But you need to give them a reason to reply back and to consider you. I doubt that many would care, initially, about your GRE, but might want to know how you've done in relevant coursework or other research. I don't think I'd ever care about the GRE specifically, though my university might. The GRE is about your general level of knowledge, not your research potential. 

But, don't send out mass emails to people along with your CV. You will be ignored. Make sure you have some knowledge of their interests and that you can speak to how your background and interests fit with theirs.