Something like this should work. Place your "GSM" field within a placeholder and then set the visibility of the placeholder based off of the "Displ_GSM" field.
<asp:PlaceHolder ID="phGSM" runat="server" Visiblity='<%# Eval<bool>("Displ_GSM") %>'>
<div>Telephone: <%# Eval("GSM") %></div>
</asp:PlaceHolder>