When strangers look at each other at the gym and attempt to remain expressionless because they are either checking the other person out or don't want to invite conversation. The former usually applies to men while the latter usually applies to women.
I know he was checking me out, but he was trying to put on his gym face.
The look of absolute focus and determination. No matter what you do in life you always have your GYMFACE on! It's an attitude - a way of life. It’s a revolution. It's the will to always train and stay 100% focused.