onBeforeUndoStack

fires before an action is added into the undo stack

boolean onBeforeUndoStack(UndoRedoAction action);

action

UndoRedoAction

a user action as an array of command objects

boolean

defines whether the default action of the event will be triggered (true) or canceled (false)

gantt.attachEvent("onBeforeUndoStack",function(action){
    // your code here
    return true;
});

Details

This event is defined in the undo extension, so you need to enable the undo plugin. Read the details in the Undo/Redo Functionality article.