The correct answer is C) Cargo holds. Cargo holds are not exempt when measuring gross tonnage, as they are an integral part of the vessel's total enclosed volume. The gross tonnage calculation includes all enclosed spaces above and below deck, with only a few specific exceptions such as the crew's quarters, navigational spaces, and some machinery spaces. The other options are incorrect because: A) the galley in a deckhouse, B) auxiliary machinery spaces above the deck, and D) the steering gear room are all exempt from the gross tonnage calculation, as they are considered non-cargo spaces.