Don't make it a gravity magnet. Make it, literally, a magnet. All substances are magnetic; some are just more magnetic than others. You can get water (e.g., a human body) to react to a magnetic field, if it's strong enough. Just put some magnets in the ceiling to diamagnetically push water downwards, and you can have people walking around on the floor.

Once you have such a system, you can also us it to cancel out the effects of high acceleration... to a degree. The limiting factor is going to be that bones have a different diamagnetic susceptibility than more watery parts, but they all react the same way to gravity/acceleration. So if you try to cancel out too much acceleration, you end up with your bones pulled one way and your flesh pulled the other.

This effect would also be present, though to a less dramatic degree, even at a normal 1 g. It wouldn't be dangerous, but it probably would be a bit uncomfortable to those who weren't used to it. Which provides you with a plot justification for flatlanders getting "spacesick" and having to "find their space legs", which might be useful for your story.