DIY & Home Improvement

If the hot and neutral wires are in a flexible metal jacket or metal conduit, and that jacket or conduit is bonded to the the metal box and at any metal junction boxes all the way back to the breaker / fuse box, then the box may be considered to be grounded and should trip the breaker if the hot wire touches the box. But if regular romex or similar style wiring is used where the conductors are in a plastic or cloth jacket, then the metal box is just "floating" and not grounded. You can still get shocked in this case.

A GFCI outlet provides extra protection when a device that is plugged into has a short to ground, then the GFCI will trip because its function does not depend on a connection to ground. But it won't protect you from a hot box.

There are GFCI breakers available that protect an entire circuit. In that case touching a hot box would trip a GFCI breaker.