Here's the GX2100 manual link.
www.manualslib.com/manual/652968/Standard-Horizon-Matrix-Gx2100.htmlThe GX2100 is an AIS Receiver. Your old Chartplotter is providing the gps input that shows the lat & long.
If you turn the GX2100 to AIS mode you should receive info on boats in your area. It'll show as a couple of concentric circles with AIS transmitting boats identified. You can set the alarm to go off if any cone within a designated distance. Check the aerial connection too.
Shipfinder is also receiving information, not transmitting. It's not just for entertainment. You don't have an AIS transmitter, that's why you don't show up on shipfinder.