Delete and Add Row In GRidview

protected void Button1_Click(object sender, EventArgs e)
    {
        DataTable dt = new DataTable();
        DataRow dr;

        dt.Columns.Add(new DataColumn("ColumnName1"));
        dt.Columns.Add(new DataColumn("ColumnName2"));
        dt.Columns.Add(new DataColumn("ColumnName3"));

        foreach (GridViewRow gvr in GridView1.Rows)
        {            
            if (((CheckBox)gvr.Cells[4].FindControl("CheckBox")).Checked == true)
            {
                dr = dt.NewRow();
                dr["ColumnName1"] = ((Label)gvr.Cells[0].FindControl("Label")).Text;
                dr["ColumnName2"] = ((Label)gvr.Cells[1].FindControl("Label")).Text;
                dr["ColumnName3"] = ((Label)gvr.Cells[2].FindControl("Label")).Text;
                dt.Rows.Add(dr);
            }
        }

        foreach (GridViewRow gvr in GridView2.Rows)
        {
            dr = dt.NewRow();
            dr["ColumnName1"] = ((Label)gvr.Cells[0].FindControl("Label")).Text;
            dr["ColumnName2"] = ((Label)gvr.Cells[1].FindControl("Label")).Text;
            dr["ColumnName3"] = ((Label)gvr.Cells[2].FindControl("Label")).Text;
            dt.Rows.Add(dr);
        }

        GridView2.DataSource = dt;
        GridView2.DataBind();
    }