The TCL/GWM widget is the default display manager in use on Starlink systems, so just logging in and initialising the Starlink Software should enable you to use the widget.
It lives in the directory /star/bin/startcl on most Starlink systems, whereas the older standard display manager lives in /star/bin. If you have /star/bin/startcl in your PATH before /star/bin, you should get the TCL/GWM widget. This is setup by the Starlink initialisation on Starlink systems.
If you want to use the older display manager, you should put /star/bin in you PATH before /star/bin/startcl, or ensure that /star/bin/startcl is NOT in your PATH.