Tell your manager about GMAT

This should be completely your call depending upon the nature of your manager and the circumstances of your employment. I would second bb's reply above. If you have a strong relationship with your manager and if he is not going to react unfavorably, you can let him know.

I informed my supervisor about the GMAT. And that was because he is a gem of a person. He had given GRE himself during his days and had got an offer from a very good university. He could not attend though because of financial and family constraints. So, he knew what I was trying to achieve and was very helpful. Not only did he support me in my endeavor, but also he gave me a glorifying LOR for applications. During my preparation too he allowed me some lax time every day to study. He would regularly keep tab of my practice test scores. When I faltered on the actual GMAT, he was very supportive.

If that's the case with you too, you can inform your manager. Else, be wary of spreading the news. After I got a wonderful score in GMAT, the news in my office spread like wildfire, and I must admit this had repercussions. Not everyone will digest/favor your pursuance of MBA. So, think over it and make an informed decision. You have to ensure that no negatives come out of you breaking the news to your manager/supervisor. Also, in case you confide in her/him, she/he should keep this to herself/himself at least until you are at a stage where you can confidently break this news to everybody (that may well be after securing an admit or the time of verification of your employment credentials post admit). Hope you get my point.