A bit of research turns up that shapefile is intentionally not listed as a supported filetype because "Shapefiles are not well defined. They're more free-form than most of the formats we support and often require additional arguments or modification to the source." -- Robert Lipe, gpsbabel author
However the same thread goes on to say shapefile is "in the
'-^3' output and marked 'hidden' for this very reason - it's not practical to treat it as 'just another format' "
Suppose I'm willing to take my chances, how does one get at the hidden formats?
xcsv is mentioned as well).