To do this right, you need the smoke detectors on their own circuit (their own circuit breaker with nothing else attached), and 14/3 (12/3 is overkill but okay if you had it). A gfi circuit is an electronic circuit that monitors the hot and neutral and if there is an unbalance, it disconnects power. The safety ground is not a direct part of the operation of a gfi. GFCI circuits are required for kitchen counter outlets, bathroom outlets, outdoor outlets, and garage outlets.
Basically, each of the detectors needs hot and neutral, and the third wire is used to allow them to be interconnected (so if one goes off, the others do, too). You should not use the ground wire for this, since if it made contact with a real ground anywhere, it would either damage the dectors or inhibit the interconnections to work...you need a dedicated, insulated wire for this.