The GT4 has direct link steering with a servo saver. Generally, if its direct link, you'll want to go metal geared since its easier to damage the servo in crashes.
Go for a high speed servo with something faster than .15 and over 50 ounces of torque at 6V should be sufficient. I use a Futaba 9402 for steering. A bit overkill in the torque department but I like it a lot.