The HS85MG is a mini servo and would not be recommended on a gasser even if it does have metal gears.

As suggested earlier, I'd recommend a servo of the same quality and size you have on your flight surfaces. You don't need as much torque for the throttle. I have used a variety of throttle servos on gassers but all are usually high quality, coreless motor, nylon geared (never saw the need for metal here) servos ... some digital, some analog. Some are even retired control surface servos or in my case heli servos that have a lot of life left in them but no longer popular for a heli application. Lots to choose from out there. All have higher resolution than a standard sport servo would have.