You get 26 ends with the GBAS system.
IF you've got 26 runway ends within the coverage area, no?
Exactly! GBAS coverage is defined by area, not the number of RWY ends.
Essentially each installation will have defined coverage and maximum use areas, some of which may be dependent on the installation itself, what's contracted for by the purchaser, terrain, non-resolvable multipath/interference issues, etc.
But it's not the ultimate solution yet - mainly because of the limited number of aircraft/FMSs that can fly GBAS and the limited number of certified systems (and hence supplier & cost options).
A useful summary can be found on
GNSS Frequently Asked Questions - GBAS
OK, it's the FAA, but it gives a reasonable overview of what's relevant globally and thereby what's available to Oz.
I reckon it's definitely the way of the future - but there's a ways to go yet before it will outdo other solutions (mainly due to cost
and breadth of usability) - even given that the annual maintenance costs are completely negligible compared with ILS.
Eventually I suspect you'll find that RNPx (AR or not) APCHs and DEPs will interface with GLS in the final stages of APCH and early stages of DEP. Even now, in some places, there are RNPx (sometimes AR) APCHs being joined to ILS Cat X finals to achieve lower minima than might be otherwise available if using only RNP AR. Not yet documented by ICAO, but it's being done.