GPS Antenna - An antenna that receives a satellite
GPS - A device that processes the satellite
into lat/long coordinates
Chart Plotter - A device that displays (graphically) the lat long coordinates
NMEA183 - A communication protocol between electronic devices
You have a Garmin 205 Chart Plotter - See my image attached.
you do not have a GPS or GPS antenna.
The 205 will take NMEA183 input - You need a GPS and GPS antenna that can produce NMEA183 output.
This GPS will send Lat/Long, speed or other data to teh plotter depending on it's capabilities.
GPS antenna is coax, NMEA183 is usually just a wire.
If the seller told you that you do not need a separate GPS he misled you.