How to duplicate the column and replace G with missing value ?

Hi All
I have the field name = GROUP_TYPE , It have G1 , G2 ...... 

I like to create another column , that is copy all the value label except "G"

So it will return as below :-

G1...G1

G2...G2

G..........

G will replace with missing value.

Can some one share with me the expression ?

Paul