How should I connect the TVS GND in a metal enclosure?
The GND of the electronics inside the enclosure. I assume you want protect a input signal ? Then the TVS should protect the signal for too high voltages relative the GND of the electronics. That needs protection.

The enclosure itself can be connected to that same GND, or via a resistor, sometimes via a RC circuit. Sometimes a resistor of 1M is used, to avoid static electricity build up.