I am wiring my GSA28 pitch and roll servos now and have done some reading on the interface with elevator trim.
One of the great things about these autopilots is that they will drive your trim tab servos to assist with control forces. So IMHO you are going to want to include a normal trim system that your GSA28 can drive while autopilot is engaged. One of the side benefits is that when you disengage AP, the airplane is already trimmed up so you don't have a big un-commanded pitch or roll at disconnect. Also, if you somehow were to get your GSA28 servos to provide a continuous elevator force to provide a trim like input, your servos would be providing a torque output all the time, which may lead to early wear and failure.
I've got a regular MAC trim servo for pitch and roll that my GSA28s drive, and when AP is disconnected, I can still trim with my stick mounted coolie hat. The regular trim system is not that expensive, and when coupled with your GSA28, really takes advantage of the full capabilities of your AP.