These technical things have subtile differences in meaning. A gangplank is something that you can move. It is just as a temporary bridge between a vessel and the shore. Your prop can be a pier or a dock or maybe a warf. English is not nearly as consistent as Deutsch. It absorbs anything that is useful and makes it its own and there are almost as many exceptions as there are rules. I wish English was as easy to spell as Deutsch. Ich ein ser kline Deutsch habe.