The ISM420 requires the use of the following circuit in the front end if you plan on using an active antenna. The circuit is not required if you elect to have no power and use a passive antenna.
RFin connects to the Antenna and the input to the GPS. In the case of the ISM420 inputs are 3.3V tolerant. So no protection needed to the GPS
We use the 0603HP – 33NXGLW inductor you can find at digi-Key.