Is there a chip capable (perhaps with a loop antenna) of sensing a 400-500 Gauss magnet at a distance of several inches?
That's a darn strong magnetic field. This should be pretty trivial to sense using any kind of magnetic field sensor.
The standard and cheap-as-hell sensor type for magnetic fields is the Hall effect sensor.
To measure movement of that field, you don't even need an IC. A magnetic field moving perpendicularly to a conductor induces a current (basics of electromagnetism!), and at these strengths, you can simply use a trace on your board or a simple wire, and any kind of amplifier to reliably sense this.
Highlights another thing: keep moving strong magnet fields away from your electronics. They induce currents in anything that is meant to conduct electricity.