GSX by default enables airstair trucks at all airports
No, it doesn't.
GSX correctly checks for a jetway to be present or not in the airport AFCAD, and if there's a jetway, it will NOT activate front stairs. The only stairs that are active on an parking with a jetway, are the rear one, which can be enabled/disabled separately.
Perhaps you are referring to those 3rd airports with fake/static jetways: they can't obviously be detected automatically (since they don't "exist", they are just part of the terminal polygonal mesh), so you will have to tell GSX about them.
I know that I can edit the airport gate(s) to turn off the airstairs, but this is a very tedious process, both for the tens to hundreds of gates depending on the airport and the hundreds of airports a user flies to.
Not really. There's no reason to edit each single parking: if you select a whole Terminal, the editor will tell you have selected many parkings at once, so your change will affect all of them.
And if you click on the root node of the airport ICAO, you'll select the whole airport, so you can fix an entire airport with A SINGLE CLICK.