ASPxClientGanttContextMenuEventArgs Class

<dx:ASPxGantt ID="Gantt" ClientInstanceName="clientGantt" >
    //...
    <ClientSideEvents 
      ContextMenu="function(s, e) {
          menu.ShowAtPos(e.htmlEvent.x,e.htmlEvent.y);
      } 
      ContextMenuCustomization="function(s, e) {
          e.cancel = true;
      } 
    />
</dx:ASPxGantt>

<dx:ASPxPopupMenu runat="server" ID="popup" ClientInstanceName="menu">
    <Items>
        <dx:MenuItem Text="Item1">
        </dx:MenuItem>
        <dx:MenuItem Text="Item2">
        </dx:MenuItem>
    </Items>        
</dx:ASPxPopupMenu>