The gym and diet system

The Extended Look shows Lucia lifting weights, with button prompts to assist the lift. Reporting on the same footage describes a weight and muscle system: eat too much and the character gains weight, neglect the gym and muscle is lost over time.

This is San Andreas coming back after twenty-two years.

Why people care about this specifically

Grand Theft Auto: San Andreas in 2004 let you make CJ fat, thin or muscular by eating and training, and it remains one of the most fondly remembered systems in the series. It also remains one of the most requested things people wanted back.

Rockstar dropped it for a reason. Body-shape systems are difficult and expensive: every item of clothing has to fit every body state, every animation has to read correctly on a heavier or lighter frame, and cutscenes have to survive a protagonist who might look wildly different from one player to the next.

Doing it for two protagonists rather than one roughly doubles that cost, which tells you how much the studio wanted it.

How it connects to everything else

It ties into appearance customisation, since body shape is the layer underneath clothing and tattoos. It plausibly ties into the evidence system too, because a physical description is a description, though Rockstar has not said so.

The reported detail about assisting the lift with button prompts is also worth noting. It suggests training is an interaction rather than a menu you visit, which is what made the San Andreas version feel like something you did rather than a stat you set.

What is not known

How far the range goes, whether it affects stamina or combat, whether food is bought at specific places, and whether the two characters track separately. All of the mechanical detail here comes from reporting rather than Rockstar.