But there are several GRE test prep websites that run on any browser, and for them it doesn't matter if you are running Linux or Windows or Mac. For example
http://www.mygretutor.com
http://www.number2.com
http://www.maxmytest.com/
These are three good ones (in order of "goodness") :); mygretutor has actual GRE computer adaptive tests, so I think that's legit! But I agree, it's lame that the GRE prep materials require windows -- I wonder if Microsoft paid them off! LOL