I've been interested in the gun magnets too, I can think of a couple of good uses for them. My only concern is will it magnatize my gun.
Not unless, as Mike pointed out, you attempted to use the magnet to magnetize your gun by repeatedly running down the length of your gun. Even at that, it is gonna take a long time.
Mike - if you try it, please post a review. As a side note, the gun magnets I've seen before were rare earth. Not sure if that one is or if it will actually hold well.