You need to use a 2-pole GFI circuit breaker.
You don't need a neutral to detect ground fault, only a circuit path that can be monitored for any current that isn't accounted for.
12 Amps out on one wire, 12 Amps back on the other...everything's fine.
12 Amps out on one wire, 11.9Amps back on the other...something's wrong with the circuit.
