The Ground Point is a Not A Terminal or Cable
The right picture show the ground connection line. Normally it installed on a metal plate of your grill, heater.
But if you do not have metal plate on your project, you can attach a metal piece or try to connect a cable to ground as well in this part.
So the igniter will work properly.
It can be worked for natural gas or propane.
Piezo igniter may need several times to push to ignite gas.