Answering to the question, no, we do not have any hours or business hours internally, and thus measuring resource utilization in hours in GanttProject is technically impossible at the time being. However, up to some extent it might be emulated using resource cost rates, depending on the ultimate purpose of using hours. E.g. if you set resource cost rate to 1, you will get the cost of a task equals to the total sum of resource working days. If you set it to 8, assuming that you have 8 business hours per day, the cost of a task will be the sum of resource working hours.