CheckBox inside GridView always Checked False when accessed on Button click in ASP.Net

There is a Gridview in my web page with select button as below:

 <div class="test01" style="height:200px;overflow-y:scroll;overflow-x:hidden" align="center">
        <asp:GridView Width="100%" ID="GAsset" runat="server" AutoGenerateColumns="False" DataKeyNames="AssetId">
            <Columns>
                <asp:BoundField DataField="AssetId" HeaderText="AssetId"/>
                <asp:BoundField DataField="AssetName" HeaderText="Asset Name"/>
                <asp:BoundField DataField="Category" HeaderText="Category" Visible="false"/>
                <asp:BoundField DataField="SubCategory" HeaderText="SubCategory" Visible="false"/>

                <asp:TemplateField HeaderText="Select">
                    <ItemTemplate>
                       <asp:CheckBox ID="chkSelect" runat="server" />
                    </ItemTemplate>
                </asp:TemplateField>
            </Columns>
        </asp:GridView>

        <asp:Label ID="LAssetMsg" runat="server" Text="Asset Not Available!" Visible="false" ForeColor="Red" Font-Bold="true"></asp:Label>

    </div>
    <div class="test01" style="text-align:center">
        <asp:Button ID="btnSelect" CssClass="button01" runat="server" Text="Select" onclick="btnSelect_Click"/>
        <asp:Button ID="btnCancel" CssClass="button01" runat="server" Text="Cancel" />
    </div>

When I am selecting any row of Gridview using checkbox and clicking on "Select" button, It is showing ch.checked= false

code is below:

        protected void btnSelect_Click(object sender, EventArgs e)
        {
            foreach (GridViewRow row in GAsset.Rows)
            {
                CheckBox ch = (CheckBox)row.FindControl("chkSelect");
                if (ch.Checked && ch != null)
                {
                    string key = Convert.ToString(GAsset.DataKeys[row.RowIndex].Value);

                    string query2 = "select AssetId, AssetName from Table1 where AssetId = '" + key + "' ";
                    DataSet ds = DataManager.DatabaseManager.GetOrCreateConnection(DataManager.DatabaseManager.ConnectionType.MySQL).GetData(query2);
                    string ID = ds.Tables[0].Rows[0][0].ToString();
                    string Name = ds.Tables[0].Rows[0][1].ToString();

                    int id = Convert.ToInt32(Request.QueryString["id"].ToString());

                    string query = "Select IdPreventive from Table2 where id='" + id + "'";
                    DataSet ds2 = DataManager.DatabaseManager.GetOrCreateConnection(DataManager.DatabaseManager.ConnectionType.MySQL).GetData(query);
                    string DamId = ds2.Tables[0].Rows[0][0].ToString();

                    if (this.DStation.Visible == true)
                    {
                        string query4 = "insert into Table3(Id, AssetId, AssetName) value('" + DamId + "','" + ID + "','" + Name + "')";
                        DataManager.DatabaseManager.GetOrCreateConnection(DataManager.DatabaseManager.ConnectionType.MySQL).ExecuteNonQueryCommand(query4, null);
                    }
                    else
                    {
                        string query4 = "insert into Table3(Id, AssetId, AssetName) value('" + DamId + "','" + ID + "','" + Name + "')";
                        DataManager.DatabaseManager.GetOrCreateConnection(DataManager.DatabaseManager.ConnectionType.MySQL).ExecuteNonQueryCommand(query4, null);
                    }
                }
            }
        }

I also tried debugging my code using break points but it is showing ch.Checked = false everytime, dont know why.

Please reply how to resolve it.