time_step

sets the minimum step (in minutes) for the task's time values

number time_step;

gantt.config.time_step = 15;
...
gantt.init("gantt_here");

Details

Start and end times of a task will have the values multiple of the time step, i.e. if time_step = 20, the task can start only at: 0, 20, 40 minutes etc.
The lightbox time selector will have the same time step.

If you want a task to be dragged with the step set via the time_step property, you need to set the round_dnd_dates config to false.

gantt.config.round_dnd_dates = false;