The only time I've had a problem with a GPS that could be construed as a rotor interference problem was in the initial certification of an FMS on a large helicopter (no names, as the problem got solved). There was a minor issue with the GPS engine in the FMS not working with that rotor speed, but both manufacturers (airframe and FMS) knew about the problem, and had decided to try it anyway. Didn't work, so changing the GPS card did solve the problem nicely.
Later in the same program, had an loss of signal when transmitting on known VHF frequencies - the avionics engineer / technician was going out of his mind trying to figure out why one comm would give the problem and not the other (you could literally watch the satellites drop off when the 'bad' comm was used).
Eventually, swapped comm boxes and both worked perfectly - the problem was traced to bad connections on the 'bad' comm box- corrosion on the terminals, evidently.
So, just as always, the mechanical connections to electronics are going to be the worst problem, not the electronics themselves.