tooltip_text

specifies the text of tooltips

start	Date	the date when a task is scheduled to begin
end	Date	the date when a task is scheduled to be completed
task	Task	the task object

Example

gantt.templates.tooltip_text = function(start,end,task){
    return "<b>Task:</b> "+task.text+"<br/><b>Start date:</b> " + 
    gantt.templates.tooltip_date_format(start)+ 
    "<br/><b>End date:</b> "+gantt.templates.tooltip_date_format(end);
};

Details

This template is defined in the tooltip extension, so you need to activate the tooltip plugin. Read the details in the Tooltips for Gantt Elements article.