Exactly what is GATTOP ? Was the carrier breaking any laws? If not, this sounds like police harassing a LAC.
Here in AZ, even if it the location is signed, you must be asked to leave before it is a trespass.
EDIT: Found this prior thread. http://forum.opencarry.org/forums/sh...ry-option-quot
Since an AR-15 is such a common weapon, and since he wasn't on a public highway, I wonder how this will play out.