Stats Displayed During Demo
A Rockstar demo revealed an on-screen menu listing multiple numeric attributes for Jason and Lucia. Rob Nelson confirmed that eating produces a substantial health buff while overeating increases weight. Gym workouts allow players to raise muscularity and strength levels, with an option for both characters to train together for shared progress.
Echoes of San Andreas Design

The attribute system mirrors the respect, stamina, muscle, fat, and sex appeal bars that defined CJ in 2004. Decisions such as diet and exercise directly altered those values and affected movement, combat, and NPC reactions. GTA 6 appears to reintroduce similar cause-and-effect feedback between player actions and physical condition.
Link to Red Dead Redemption 2 Systems
Running occasionally triggers a cardio workout notification, and food items carry heart or lightning bolt icons identical to those used for health and stamina cores in RDR 2. Sleep is also tracked, suggesting rest will influence recovery. The presence of calories, fat, and muscle values indicates nutrition will determine whether characters remain in peak condition or suffer penalties.