I looked up the G-shield, and you're right - it seems to have 3 drivers built in, so you're okay there. But it maxes out at 24 volts, which is marginal for driving NEMA 23 motors (forget about 12v). Most of them will have too much inductance to run well at that voltage, so look for motors with as little as possible. (.87 mH would be optimum, but anything under 1mH should work okay - don't worry about the holding torque number).Here's an example of a motor like that:
Stepper Motor, NEMA 23, 2.3V VEXTA PK266-03A-C22 | MPJA.COM