Hi Kamil
1. If you simply want to change all instances of 'Due' in the entire UI to something else eg 'MyDue' you can do this by creating a custom translation file and translating all references to 'D/due' to something else.
2. If you only want to rename 'Due' it in the Gantt view this can probably be done the same way
3. If you want to create a new custom attribute and use this in place of the Due date in the Gantt View then this cannot currently be done.