public static long gcCodeToGcId(@Nullable final String gccode) {
        return codeToId("GC", gccode);
    }
Convert GCCode (geocode) to (old) GCIds
<br>
For invalid gccodes (including null), 0 will be returned
<br>
Based on <a href="http://www.geoclub.de/viewtopic.php?f=111&t=54859&start=40">...</a>
see <a href="http://support.groundspeak.com/index.php?pg=kb.printer.friendly&id=1#p221">...</a> (checked on 13.9.20; seems to be outdated)
see for algorithm e.g. <a href="http://kryptografie.de/kryptografie/chiffre/gc-code.htm">...</a> (german)
main/src/main/java/cgeo/geocaching/connector/gc/GCUtils.java