It depends on the meaning. Geadh meaning an iron rod or gèadh meaning goose or goose egg. To break it down for gèadh, g - (as in gate, get), è - as a long a, a - (short, as in bat, hat, etc.; long, as in gate, mate, etc.), dh - gh and y (gh as the ch when in contact with a). So gaach for gèadh and gahch for geadh.
I am not a native speaker. This is just my interpretation from "A Gaelic Grammar" by George Calder, 1990 by Gairm Publications.