Hello,
The following article has an example of adding custom buttons and attaching the click event to them:
https://docs.dhtmlx.com/gantt/api__gantt_quickinfo_buttons_config.html
You can use the same approach for the default buttons:
gantt.$click.buttons.edit=function(id){
gantt.message("Custom function");
return true;
};
Here is the snippet:
http://snippet.dhtmlx.com/5/3502823f2