It describes a person who tends to be more comfortable and loves being at the Gym. Gymtroverts sometimes spend all day at the gym, often think about going to the gym and basically want to live at the gym. The feeling of being at the gym energizes them more than other people.
When I took my three year old with stomach pains to the pediatrician, she referred me to a gastroentomologist after learning that he puts everything in his mouth when playing in the yard.