Okay so I'll start by apologizing for the length of this reply but I can't explain it any shorter I think, so sorry for the wall of text.
Ah, well I guess "gamepad_test_mapping" has to be a "new" function (new as in GMS2, I was using GMS 1.4 for the majority of making this game) since I don't think I've seen that one before.
So in theory I could do what you asked but it would require me to rewrite my controller support code which I'm not sure I want to do.
And even if I did, this only potentially solves the problem for a number of controllers since every single controller is different.
This right here in the GML manual:
"however due to the huge number of controller types and brands out there, it is impossible to map the GML constants to the correct inputs for every make and model"
This is exactly what I've been trying to tell people when they ask why I can't detect the name of what button is being pressed for example, because the same button has different ID's on different controllers.And the issue you talk about is similar to that in which I have to manually add every single controller to a list and detect which one it is, which simply isn't feasible to me.
I know you only asked for a couple of controllers to be fixed this way but if I start doing a few then I have to do others you know (and rewrite my controller support as mentioned) and I'm not sure I'm up for that, depends on how big of a problem it is later.
And regarding "InputCandy", I won't be using any external libraries for various reasons, so that won't help.
Sorry if this is not exactly the answer you wanted but it is what it is for now, I'm sure I'll work on better controller support in the future but not right now.