I followed your instructions but have error 

qty is not null when i am using textbox it is working fine but when i used label then it is giving error

Object reference not set to an instance of an object.

Line 333:
Line 334:                TextBox lbl1 = (TextBox)e.Row.FindControl("QTY");
Line 335:                TotalValue1 += Convert.ToDecimal(lbl1.Text);
Line 336:            }
Line 337:            else if (e.Row.RowType == DataControlRowType.Footer)

on below link

Enable TextBox on GridView row edit Button Click using C# and VB.net in ASP.Net

I am doing column total of qty 

        protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
        {
            if (e.Row.RowType == DataControlRowType.DataRow)
            {
                TextBox lbl1 = (TextBox)e.Row.FindControl("QTY");
                TotalValue1 += Convert.ToDecimal(lbl1.Text);
            }
            else if (e.Row.RowType == DataControlRowType.Footer)
            {
                Label lb1 = (Label)e.Row.FindControl("lblTotal1");
                lb1.Text = TotalValue1.ToString();
                txtconqty.Text = TotalValue1.ToString();           
            }

            ///disable qty textbox

            if (e.Row.RowType == DataControlRowType.DataRow)
            {
                TextBox txtqtygv = (e.Row.FindControl("qty") as TextBox);
                txtqtygv.Enabled = true;
            }