davegravy & massahwahl
Your alarm panels most likely send a voltage down the wire to the sensor and then back to the panel. Somewhere in that line (ideally at the sensor) is a resistor that alters the signal in a way that is recognizable to your panel. If there is a line break, or the resistor gets damaged, or the sensor is tripped (technically also a line break), the alarm is tripped. So if you want to use the GC-100, you'll need to program it to do that.
There are other forums around that are dedicated to DIY security alarm systems, filled with folks in your situation (old alarm system, trying to convert to diy). They may be able to give you a leg up on voltages, resistors, manuals, potential interfaces, documentation, etc. Good luck.