If the gravel road is packed and relitivly smooth I don't see why a "road" bike wouldn't work. I think a cyclocross bike woudl be cool (imho). I routinely ride my aluminum specilized Allez on gravel trails, but it's usally no more than a 1/4 mile or so. Being able to have the option of a much larger (35mm) tire would make the cross bike a nice choice.

I would also recommend some tires like the Conti gators or specilized armadillos that are puncture resistant.

Another thing, maybe if you are not sure, buy something used.

Good luck.

Jeff