Working Day
Using days to specify the time a task takes is interpreted by Get Gantt as meaning 'your' working day.
The chart settings specify the working hours in a day. Tasks taking days will consider each working day to last the specified number of hours and measure the task's total time accordingly.
For example, if the chart settings state working hours are 2 per day, a task requiring 4 days will be considered to last 8 hours.
If greater granularity is required, specify the task in hours instead of days.
Specifying 2.5 days or 18.2 hours may also provide greater precision.