How to find a task/activity by it's ID?

// Returns the Gantt Model's internal activity/task provided an id.

var ganttControl = $('#gantt_container').data("GanttControl");

var activity = ganttControl.Model.GetActivityById(id);

And if you want to retrieve the bound data item in your data source, you can do this:

// Task is the bound data source item

var taskSource = activity.DataSource;