There are (at least) two ways of assigning a hotkey to a gadget in gtk. Either when you create the gadget (in which case gtk does not let you change it in any way whatsoever after creating it), or by using the above mentioned gtk api. So yes, it's painful, but unfortunately it appears to be necessary to duplicate behaviour on other platforms.
OK, well couldn't we just get GTKMaxGUI to free the gadget and recreate an identical one without a hot key? Would that not be easier and would avoid all this low-level accelerator map stuff?