It should be in the 'point' menu of GSD. You've probably already expanded it to no avail.
The 'Extremum' command does not come with all versions of V5 GSD. My ex-employer had the 'Express' setup and did not have 'Extremum' until they upgraded to the HDX & HCX configurations. Missing, too, from the basic licenses are some 'Sweep' sub-options, 'Adaptive Sweep', 'Smooth curve' and the Federation sub-option of the 'Join' command... the absence of all these is lamentable if you've been used to having them.
I'm willing to be told otherwise, but I think a macro can still use a command that's unavailable normally in a workbench. I came to this conclusion because I use an old 'bounding box' VBA macro that created extremums that I could not edit or copy outside of the macro.