I need an idiots guide on how to change the GSX default gate numbering, because everything in the airport customisation/gate selection options seems to be Gxx ........
It's not the GSX default gate numbering which is Gxx, it's the numbering of the default scenery in P3D V5, which in lots of airport has a lower quality database than FSX or even P3D4 had, I guess because they are old, but were originally licensed from better data from Jeppesen back in FSX days. P3D used that data until V4, then it switched to more updated data in V5, but not as detailed as it used to be.
GSX, of course, will just show what's in the scenery AFCAD so, if you have better airports (usually 3rd party), you'll see more realistic gate assignments.
GSX DOES have the ability to show gate names and groups with completely different names than what is found in the AFCAD, which has several limitations and is quite inflexible with naming, but this feature is only available for FSDT sceneries, since a possible UI to define new parking names and groups and associate them with the default parking names and groups is just not there, we can do it only at the programming level, for our sceneries, which have more flexibility than the standard GSX customization, since they all come with custom Python scripts than can basically do everything.
This was never a big issue because, until P3D V4, the default database was *reasonably* accurate when parking names are concerned and, since most users prefer 3rd party airports, we never thought of exposing this feature to user customization, you only noticed it now, because the V5 default database is worse in this aspect.
We'll keep it in mind for future upgrades.