By default all nodes in the Gantt chart are expanded. You can override it for each node (task) by setting task.row.collapsed = true:
dp.tasks.list = [
{
id: 1
start: "2018-01-01T09:00:00",
end: "2018-01-01T13:00:00",
text: "Task 1",
row: {
collapsed: true
}
}
];