|I now understand both why gs is not printing FC debug messages and why it is not matching the same fonts that fc-match does.|
I downloaded the Centos 7 ghostscript source package tonight and found that the only part of fontconfig that it uses is the subset of routines to collect the names of fonts available. It adds the FC font list to its own set of fonts, and it then does its own font searching and matching.
So the font matching problem is somewhere in gs, not fontconfig. Thanks for the responses; I hope knowing that gs’s use of fontconfig is limited comes in useful.