The Gym can be found by clicking Gym in the left-hand sidebar or by clicking the Gym icon in the top bar whilst on mobile.
The Gym is where every narco goes to train their Battle Stats. You can spend Energy in order to increase any one of the 4 main BattleStats which will assist you during attacks against other players or whilst defending.
Battle Stats in Cartel Empire grow exponentially, this means that the more Energy you invest in any one BattleStat, the quicker it will grow during subsequent training.
Gym gains can be increased from a variety of sources including Cartel perks and University courses. Gym gains are impacted massively by Intelligence which is a multiplier on how much stats you earn.