I don't think the word is a borrowing from Dutch (the Dutch word for this object is loopplank), but its roots are Germanic and reflect the common origins of the Dutch and English languages. It's a compound word of gang and plank, similar to gangway and gangboard. The common root is the word gang, which can be found in most Germanic languages. The Oxford English Dictionary explains the connections:

The word is cognate with the Old Frisian gong, meaning "way, journey, travelling", or "degree of relationship, chronological sequence"; the Old Dutch gang, meaning "entrance, step"; Middle Dutch gang, or "going, walking, gait, ability to walk, movement in a particular direction, travelling, journey, passage, gangway"; Old Saxon gang, meaning "walk, way, course"; Middle Low German gang, meaning "going, walking, gait, ability to walk, course, journey, route, passage, gangway"; Old High German gang, meaning "going walking, travelling, journey, step, stride, passage, privy"; Old Icelandic gangr, meaning "going, walking". Similar terms exist in Swedish and Danish. You'll still find cognates today in verbs such as gehen in German and going in English.

The OED dates the first use of gangplank to 1785, but the words gangway and gangboard were used earlier. There is a record of the Dutch word gangboord, then spelled gangboort, from 1643, denoting part of the deck of a ship. So it's possible this term was borrowed from the Dutch, but it more likely reflects the common Germanic origins of both languages. (And, in case you were wondering, the Dutch word for plank is exactly the same as the English.)

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