you may use RowDataBound even of Grid view.
this event is fired when your datasource is bind with grid view. also this event is available for each of row.
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
if(e.Row.RowType == DataControlRowType.DataRow)
e.Row.Cells.Text = Convert.ToInt32(e.Row.Cells.Text) + Convert.ToInt32(e.Row.Cells.Text);
replace your original column id with this example
hope this will help you.