I believe the problem is two fold, which gps 120 do you have, nema or seatalk
If it is sea talk the platter does not power
the sea talk connection, so you have to get 12 volts to the gps thru a junction box. If you are using a nema gps then you still need to get power
to the gps.
Take a look in the raymarine manuals
, it is spelled out there.