Gyms
GTA 6 has gyms where you can work out, Rob Nelson of Rockstar North told IGN, and IGN reports that going to the gym together trains both Jason and Lucia.

“We do have gyms where you can go and work out”
Rob Nelson, co-studio head at Rockstar North, told IGN that GTA 6 has gyms and that eating has a dramatic effect on your health. According to IGN, overeating adds weight, and going to the gym together trains both characters. Gyms are also on the list of side activities in Game Informer's Sept. 29, 2026, cover story, IGN reports.
IGN reports seeing on-screen stats for height, weight, sleep, nutrition, calories, muscle, fat and strength. That list is IGN's observation of the build it was shown, not a Rockstar statement, so the exact stats are not confirmed.