GLS in a Boeing is almost identical to an ILS with almost no training differences, except that it is a whole lot better . No beam interference and one station covers up to 30nm radius . And Cat III will be certified soon . Cost might be an issue now but I’ll bet there will be cheap MMRs for GA aircraft sooner rather than later.
If you had the choice why would you spend money on an ILS ?
Problem with that argument is that the technology is already 25 years old and the CAT III was supposed to be around in the mid 2000s. Now here we are in 2020, with only late model airliners fitted with the equipment and the Nav Aids only found in Sydney and Melbourne I think you will be waiting a long time for your cheap GA unit.