Doing an internship is not going to help you in any way towards your preparations for the GRE/GMAT. However, doing an internship will definitely help in building up your profile. Considering that you are taking up these exams, I am assuming that you are looking forward to take admissions in foreign colleges/universities, these institutions give a considerable amount of weightage to your non academic activities. So in order to make a strong profile you definitely need do an internship before you start applying to these universities/colleges.