I stated previously that the G/S is reliable all the way to touchdown in a CAT II ILS. It may or may not be reliable below DA on a CAT I ILS. There is no required margin below DA, at least in the U.S.
If you have been qualified on fail-active CAT III autoland you should have been trained that it is mostly RA that is controlling the vertical path below 100 feet.
As to CAT I, at least in the U.S. you are supposed to rely on visual cues below DA. Those fortunate enough to have a modern HUD, have a calculated vertical path that is displayed with the runway symbology below DA.
If the runway has a VGSI and you can see it, so much the better.
In the 1960s and 1970s the FAA falsely asserted that approach lights provide vertical guidance. After a couple of low visibility accidents they backed away from that assertion.
My own experience was that I always saw the first part of the runway at DA on a CAT I ILS. The runway perspective, even a small portion does provide a vertical sight reference.