Firstly I think you are asking for a "LastGamepadIndex" expression? The gamepad ID is a string describing it, which doesn't sound especially useful to this case.
Secondly I don't see how it helps by itself: currently you have to always specify a gamepad index when using a trigger like 'On button pressed', so the expression would be redundant, it would always be the same as the number you already provided in the trigger. So are you in addition to that asking for new triggers which don't specify a gamepad index and provide it by an expression instead? That did not appear to be specifically outlined.
Thirdly the way conditions work, that could not really be extended to the non-trigger 'Is button down'. If you have 'Is button A down' that is true for any gamepad, then if two gamepads press A simultaneously, then the condition still runs its actions once with a LastGamepadIndex for just one of the gamepads, since that's just how the event system works. So we could extend new gamepad-neutral triggers, but not non-trigger conditions, which then makes a sort of weird inconsistency between what you can do without a gamepad index and what you can do with one.
This is why you should always describe feature requests in as much detail as possible, a bit like the approach with bug reports. The suggestion just raises more questions and has implications you probably didn't foresee.