GLS CAT III would have been certified several years ago if the FAA was interested.
A cursory search on the internet will illustrate that while the FAA have pretended to be slightly interested, they really never have been. Perhaps this is “supply and demand” capitalism at it’s finest.
Current FAA policy is that they are not interested in FAA GLS installations. Private airports may install GLS at their own expense. A small few have.
This is why GLS has not become the “go-to” approach for the future that commonsense dictated that it should. This is why many IFR aircraft manufacturers are not even installing GLS capabilities as standard equipment. Boeing, to their credit, have been offering GLS capable equipment as a “no-cost option” for many years now. My fleet has it on all B737’s since 2002. Airbus, not so much? I think the latest Airbus types might have it: I think the A380’s do. Maybe A350’s. Can anyone confirm? Latest bizjets?
From a technical perspective, any new aircraft fitted with GPS and ILS should be able to achieve GLS standard without further complications. The data is there, and the antennas are there. The only complication is CPU processing, presentation, and certification. But this is aviation, right?
Australia chose to install their first GLS in Sydney! Great... an airport that already had 6 ILS’s. Ok, maybe it was a proof of concept test rig. Fine. Next installation: Melbourne. Also not helpful for aviation, other than a lower viz for runway 34, which is rarely helpful in practice. It was, however, a useful demonstration of the power of the GA community when it had a small impact on airspace over Port Phillip Bay.
The whole point of GLS is to provide a cheap and certified accurate precision approach to an airport without the need for local antenna installations for localizer and glideslope. And once you have it for one runway, you can have it for all runways, even including other runways at other airports within a certain radius.
It HAS to be the future... but it seems the future is a long way away.