As far as I know, the GPS antenna beeing located under the main rotor doesn't seem to be too much affected ; on our S365's, the antenna is normally located on top of the tail fin, but on one of them it's just on the cockpit roof. We never noticed any difference on a normal use with the same equipment (trimble 2100 series).
Besides this, I have seen of a full loss of signal on a GPS antenna located on the tail boom of a AS332, but the reason was a thin layer of carbon from exhaust fumes...