Question

How to label a vertex as G^2

Answer 1

Click on the vertex to select it, then press F2 to start yEd's inline label editor, enter "G^2", and press RETURN.

Answer 2

To get G² as a label, edit the label and type the 'G'.  Then enter ALT+0178 to get the superscript '2'.  More specifically, hold down the ALT key and then type 0178 on the numeric keypad.

Comments

That will only work on Windows systems, will it not? If you need a superscript (which the original question does not clearly ask for), then using HTML formatting is the better option.