SetTaskTitlePosition(
    position: ASPxClientGanttTaskTitlePosition
): void

<script>
     function click(s, e) {
          clientGantt.ShowTaskTitlePosition(ASPxClientGanttTaskTitlePosition.None);
     }
</script>
...
<dx:ASPxGantt ID="Gantt" runat="server" ClientInstanceName="clientGantt" >
    //...
</dx:ASPxGantt>

<dx:ASPxButton ID="ASPxButton1" runat="server" Text="Clear" AutoPostBack="false">
     <ClientSideEvents Click="click" />
</dx:ASPxButton>