it all depends on the terrain, speed, strength and accuracy you want for the turret.
an rc car/truck could work for the main chassis with pan/tilt servos with brackets mounted on them...
you will most likely need to make a custom bracket to hold the gun to the servo.
you will also need a firing mech to pull the trigger.
so tell us, what are the specs of the gun? weight, size, fire rate?
and how fast do you want it to go and what terrain?