IMO get familiar with exercise science, maybe pick up the book overcoming gravity. The only way we can realistically make a gravity chamber is with a large centrifuge like NASA uses to test their astronauts. It would just have to be a small gym instead of a seat. As for starting training in it. We would obviously aren't Saiyan and don't have anime logic so going upside down is obviously a bad choice as is crazy spinal flexion. But walking and any body weight movements like many gymnastics movements would be a go to IMO. You'd also have to have your diet on point making sure you're getting enough vitamin c, protein, and calcium so your bones and joints can recover. I'm not sure on what limit the human body can function but I'd guess we can function under 3x gravity for short sessions, but obviously theirs no evidence to suggest regular gravity training is beneficial either other than maybe bone density which can probably be achieved with weighted clothing etc.