The ABC News article President Trump prompts state of confusion over space policy with tweet covers POTUS' tweet and NASA director Bridenstine's clarification tweet a few hours later, as well as cites this evidence of the "intertwination" of the Moon (or at least the Gateway) and crewed missions to Mars by quoting NASA's page Moon to Mars Overview:
Exploration of the Moon and Mars is intertwined. The Moon provides an opportunity to test new tools, instruments and equipment that could be used on Mars, including human habitats, life support systems, and technologies and practices that could help us build self-sustaining outposts away from Earth. Living on the Gateway for months at a time will also allow researchers to understand how the human body responds in a true deep space environment before committing to the years-long journey to Mars. (emphasis added)
I assume that "how the human body responds" excludes the overtly psychological issues of spending months in a tin can, and focuses on more biological and medical issues, though these can never viewed as completely separate.
Question: What are the aspects of "a true deep space environment" available on the Gateway that have not been thoroughly explored by decades of habitation by hundreds of astronauts in all of the crewed stays on nine space stations already?
Is this simply an excuse to radiate human subjects without exactly saying so?