Hi,
in this API call you can't use UNC paths. As stated in the documentation, you have to use the printer names as they are returned by the command line utility $GDCDIR/bin/printerinfo.
This command lists all local printers. If you want to print to a remote printer then install it first locally thereby referring to the UNC network path. Then printing to such a "local" printer will result in printing to the remote printer.
Have you considered using server side silent printing for this case (fgl_report_selectDevice("Printer"))? This can simplify things since the remote printers are defined only once on the server and it works also for other display clients (not only GDC).
Regards,
Alex