So you have a new servo never used but somehow the gear is broken?

That just does not add up. Obviously the servo was used and either setup incorrectly or the vehicle hit something. Metal gears just don't strip on their own. In any event you do have a 2 year warranty and on this servo (MP or MK gear train servos) we do warranty the gears against breaking. What you need to do is send it to service with a copy of your receipt for warranty coverage.
And as Tres pointed out the 56xx series servo is on the bottom end of what would be recomended for an 1/8 scale vehicle. 1/8th scale is very brutal on servos so a high quality servo like our titanium geared servos from the get go will keep you running much longer before having to rebuild/replace your servos.
And you are using one of the horns that came with the servo or one made for a Hitec servo correct? Every servo manufacturer has a different spline count so they are not interchangable.