Just an FYI: I plan on taking the GRE at a test center (in India) so I called the center where I intend to take the exam and I was told that I need to have a mask on for the entire duration of the test. So, for anyone planning on giving the test, anywhere in the world, there is a good chance this rule will apply at any other testing location too.

I gave a practice test with a mask on and needless to say it brought my score down by a few points. So, to try to minimize the effects of wearing a mask I have been studying with a mask on to get used to the feeling of having something on my face. If you use eyewear then you have to consider the effect of foggy glasses when you exhale. I will be giving another practice test soon with a mask on, hopefully, that goes well!