How to group specific column in grid view

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ASP.NET

HTML

C#3.5

Gridview

In Grid view I am having one field Items Group. I want to group all the Item group values together in gridview row wise with header name as Itemgroup. Like Header Text as Itemgroup values followed by its values respectfully.

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" HorizontalAlign="Center">
    <Columns>
        <asp:TemplateField HeaderText="Item Name">
            <ItemTemplate>
                <asp:Label ID="lblName" runat="server" Text='<%# Eval("TestItemName") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Items Group">
            <ItemTemplate>
                <asp:Label ID="lblGroup" runat="server" Text='<%# Eval("TestItemGroup") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Value">
            <ItemTemplate>
                <asp:TextBox ID="txtItemGroup" runat="server" Text='<%# Eval("ItemGroup") %>'></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField Visible="false">
            <ItemTemplate>
                <asp:Label ID="lblGroup12" runat="server" Text='<%# Eval("TestItemID") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="">
            <ItemTemplate>
                <asp:Label ID="lblItemValue" runat="server" Text='<%# Eval("TestItemValues") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Default Values">
            <ItemTemplate>
                <asp:Label ID="lblDefaultValues" runat="server" Text='<%# Eval("DefaultValues") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView> 

Binding Grid view in .cs:

public void Foo()
    {
        int ID = Convert.ToInt32(txtID.Text);
        sqlstr = "select * from Test_Items where TestID = '" + ID + "'";
        SqlDataAdapter da = new SqlDataAdapter(sqlstr, con);
        DataSet ds = new DataSet();
        da.Fill(ds);
        if (ds != null && ds.Tables[0].Rows.Count > 0)
        {
            GridView1.DataSource = ds;
            GridView1.DataBind();
            btnSend1.Visible = true;
        }
        else
        {
            ScriptManager.RegisterStartupScript(this, GetType(), "", "alert('Invalid ID (or) No Data to Display..!');", true);
        }
    }