Get SelectedIndex of DataGridViewComboboxColumn

implement this functionality in your event....

            foreach (GridViewRow gvrow in gviewID.Rows)
            {
                Combbox cmbbox = (Combobox)gvrow.Cells[0].FindControl("cmbboxID");
                if (cmbbox .Checked && cmbbox  != null)
                {
                  //your code goes here
                }
            }

hope it help you..........