Thanks to the many people who sent their views on this. To summarize
1. both the VMS and UNIX versions of GCG seem fine in terms of performance,
ease of use etc.
2. it is recommended that you go with the one where you have the best
3. regarding choice of OS, external considerations seem to matter more
than merely the running of GCG; e.g. the experience of your users,
other software you want to run, how UNIX or VMS fit into the rest
of the environment at the site.