var q = from x in Table1 group x by x.GroupId into g
select new{Key = g.Key , Group = g}

foreach (var g in q)
    {
         foreach (var n in g.Group)
        {
            Console.WriteLine(n.Name);
        }
    }

<form id="form1" runat="server">
    <div>
        <asp:Repeater ID="Repeater1" runat="server" OnItemDataBound="Repeater1_ItemDataBound">
            <ItemTemplate>
            Gruop <asp:Label ID="LabelGroupId" runat="server" Text='<%# Eval("GroupId") %>'  />
                <asp:GridView ID="GridView1" runat="server">
                </asp:GridView>
            </ItemTemplate>
        </asp:Repeater>
    </div>
    </form>

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            var q = from x in GetData()
                    group x by x.Group into g
                    select new { GroupId = g.Key };
            Repeater1.DataSource = q;
            Repeater1.DataBind();
        }
    }

    protected void Repeater1_ItemDataBound(Object Sender, RepeaterItemEventArgs e)
    {

        var grid = e.Item.FindControl("GridView1") as GridView;
        var id = e.Item.FindControl("LabelGroupId") as Label;
        if (!string.IsNullOrEmpty(id.Text))
        {
            var data = GetDataById(Int32.Parse(id.Text));
            grid.DataSource = data;
            grid.DataBind();
        }

    }

    private IEnumerable<Rec> GetDataById(int p)
    {
        return GetData().Where(x => x.Group == p);
    }
    private IEnumerable<Rec> GetData()
    {
        for (int i = 0; i < 9; i++)
        {
            yield return new Rec
            {
                Id = i,
                Group = i % 3,
                Caption = "Caption" + i,
                Text = "Text" + i,
            };
        }
    }
    class Rec
    {
        public int Id { get; set; }
        public int Group { get; set; }
        public string Caption { get; set; }
        public string Text { get; set; }
    }
}