How is a cubic root done in gnome-calculator?

Solution 1:

The n-th root is calculated when you put a subscript number before the square root sign.

₃√27

To enter subscript numbers with the keyboard hold down Alt while entering the number.

E.g.

1. Go to Advanced mode
2. Press Alt while typing 3
3. Press √

and so on

Solution 2:

Open Calculator and goto Help>Contents (or simply press F1), You will find help for Powers and Roots:-

Where you will find information that:

n-th roots can be calculated by putting a subscript number before the root sign.
₃√2

The superscripting and subscripting numbers can be put by holing Ctrl and Alt respectively!.