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.
Good luck.