Heading hold gyros measure the signal width at boot up of the channels they control, that is the neutral (they don't assume 1520us because not every radio centers the same).

Anything after that measurement is presumed to be stick input. Change the trim and reboot the gyro or change the sub trim and reboot the gyro, then that becomes the new center measurement.

Problems arose when guys had different trims for different flight modes, boot the gyro in normal mode, take off and flip to idle 1, Idle 1 rudder trim is different and the gyro took it as a command to turn, the result was it looked to the pilot like the tail was "drifting" when in fact it was doing exactly what it was asked to do.