for finding cell values of that row you need to find
container row of button first.
See
THIS[
^] will surely help you.
[update]
As you now got object of GridViewRow(grdRow) from linked article.
you could further investigate like below to find datakey on Button click event
GridViewName.DataKeys[grdRow.RowIndex].Value.toString();
you do not need to loop through each row if button is present for every row.