Objective:
I would like to use a different geoid when using Geoid height (geoid.gx) to calculate the height of the geoid for my x and y locations.
Resolution:
The geoid.gx will create a channel of the geoid height relative to the GRS 80 spheroid for a given pair of coordinate channels. A few geoids have been provided with the software and these are essentially grid files that are sampled at the given X and Y locations when the GX is run.
To create a custom geoid, create a grid file that contains the elevation of the geoid in metres relative to the GRS 1980 ellipsoid. The grid must have a defined coordinate system. When running geoid.gx, select your geoid model as "Other grid" and you'll be able to select a custom grid as your geoid model.