Arduino turning a knob left and right with Digital PWM
You could consider using an RC servo. You will want one that will rotate an appropriate number of degrees (maybe 240 or 270). Most only rotate 90 or 120 degrees. One possibility is a Henge MD260, but shop around. 

The interface is simple- a 1msec pulse every 20msec causes it to rotate to one extreme, and a 2msec pulse every 20msec causes to rotate to the other extreme, 1.5msec puts it in the center (sometimes these times vary a bit from servo to servo). 

You don't have to 'home' it as a stepper (and gearbox, most likely) would require. An RC servo has a built-in feedback potentiometer connected mechanically to the output shaft, so you control absolute position rather than incrementally moving it from wherever it happened to be last (perhaps after the power was interrupted). 

If you can live with 180° rotation, there are more choices, and you can get ones with a standard spline which fits shaft couplers to a 1/4" (6.35mm) shaft.