They're talking about living inside an asteroid that has been moved to the lunar gateway. You can imagine a looped track dug inside, to provide simulated gravity. I don't think that's very feasible, I prefer the "Islands in space" idea from Gerard O'Neill. To provide artificial gravity.

https://en.m.wikipedia.org/wiki/O%27Neill_cylinder

That must be a giant rock that you must carve an internal loop out of.

And then, how much energy would it even take to spin such a huge rock? I think you’d need a new exotic propulsion system just to rotate it. Maybe nuclear or a sustained ion propulsion.

Sorry I wasn't clear.

Being beneath the surface provides radiation protection.

Specifically by digging out tunnels beneath the surface of moderates size asteroids, putting down butyl ruber tubes, and laying tracks within. By having the cars on the track go around the curved track an artificial gravity is induced.

But you can stop the space train cars, step outside, and there's no deep gravity well to make getting things to and from there so expensive.

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I'd also like to make it clear that the Asteroid Retrieval Bus, the mission that was canceled, is the reason the lunar gateway concept exists. Having the lunar gateway but not asteroid de-spin and retrieval is putting the cart before the horse without intentions of getting a horse.

https://en.wikipedia.org/wiki/Power_and_Propulsion_Element#A...

>The Asteroid Redirect Vehicle... ...mission was cancelled in early 2017 and the spacecraft's propulsion segment became the Power and Propulsion Element (PPE) for the Deep Space Gateway, now known as the Gateway.

I don't know exactly what the OP means but putting something big and sold between it and the sun is a good way to limit radiation. Maybe the moon in this case?

The earth's magnetosphere protects from radiation.