I tried installing the latest stable version of Gizmo and the problem remained.
Now here's the good news ...
I stumbled on the latest beta version, as announced here: https://en-gb.facebook.com/G64xpl/posts/1799850900304464
Installed it, and Gizmo is up and running!
It turns out that this latest beta of Gizmo also fixes an issue I spotted on the MCP where the IAS and Heading values were not rendering correctly.
I have to say it's not been the nicest unboxing experience. 2 defects in under 5 minutes doesn't fill this customer with confidence!
If X-Aviation are open to constructive feedback then I would remind them that a common practice for avoiding such situations would be for the publisher's QA team to perform shakedown testing on all *candidate platforms*, after which they could declare with some degree of confidence a shortlist of *supported platforms*. A supported platform is simply an environment in which the application installs well, performs well, and continues to perform well with the minimum of fuss. If this were the case for my purchase of 737 Classic, and if "Sierra OS" were absent from the list of supported platforms, then I would have thought twice about purchasing.