Ok, got it but still not perfect.

For rows 1 to 9 enter: =REPLACE(A1,8,2,ROW()+1) and drag down.
For rows 10 and above enter: =REPLACE(A9,7,2,ROW()+1) and drag down.

Not sure why it is adding an extra digit if I am starting the check at the 8th character for rows with more than 2 digits (i.e starting at row 10 and above).

This will work assuming your text is entered as "GSLAB-xx"