Resolution:
The GDI portion of RTmon (rtmongdi) has been deprecated as of SmartSockets 6.8.0 [GA] with the exception of the Microsoft Windows platform. The Windows GDI will also be deprecated in a future release of the product. (It is still available as of SmartSockets 6.8.1[GA])  
Where the GDI no longer exists you now must use the command line interface.
(NOTE:  Using the Monitoring API the developer can build a graphical or browser based interface which meets their specific needs)

$rtmon -help

SmartSockets RTmon 6.8.1 [GA]  2008/01/04 09:23
Copyright (c) 1991-2007 TIBCO Software Inc.. All Rights Reserved.
For customer support please email support@tibco.com
Usage: rtmon [ -command &ltfile_name> ]
                 [ -help ] [ -nofork ] [ -runtime ]
                 [ -xrm &ltresourcestring> ]
             [ -username &ltusername> ] [ -password &ltpassword> ]

$ rtmon -runtime
SmartSockets RTmon 6.8.1 [GA]  2008/01/04 09:23
Copyright (c) 1991-2008 TIBCO Software Inc.. All Rights Reserved.
For customer support please email support@tibco.com
Type 'help' for help.
MON>

You can always use an older version of SmartSockets on a separate node to use the RTmon GDI but it will not be formally supported.