Having this implemented in PGC would be very nice, but uploading and parsing large GPX files for getting basically only the GC codes to import is quite waste of bandwidth.
Getting GC codes from GPX file is very simple procedure (you can probably get a list of gc codes from your application), but to convert you need just simple transformation, which should be pretty easy to run on every Linux/Mac, but there should be a way on Windows too. I just run
xsltproc gpx2gccode.xsl GPX_FILE