I hope you didn't pay too much for that machine. It seems that Gorilla CNC went bankrupt in 2009, leaving warranty holders out of luck. You can read about this saga here: https://www.cnczone.com/forums/goril...rs-owners.html if you haven't already. Even when the company was running, it seems there were a lot of problems with these machines, only some of which were addressed. You might try asking General if they have any manuals for them, but I wouldn't be too hopeful. Maybe someone here still has one. Does it seem to work at all? If not, the best thing to do might be to treat it as a frame and add your own new controller, ditching the old box and all its issues. You might be able to reuse the motors, though.
If you don't see another E-stop button on your machine, that's probably the "button of sorts" you noticed. If there's a metal plate with wires going to the controller, that's probably a touch-off plate. Look for a metal clamp with wires somewhere nearby. Usually you'd put the clamp on the spindle nose and lower the tool until it hits the plate and completes the circuit, establishing Z zero.