What does the roughly 250,000 gross tonnage listed for Icon of the Seas actually measure?

Answer: It measures the total amount of enclosed space inside the ship.

It measures the total amount of enclosed space inside the ship. ✓ — Right. Gross tonnage is calculated from the total volume of enclosed spaces, then adjusted with a logarithmic factor; it is not a weight. The word tonnage makes the weight reading feel almost right, because shipping also uses displacement for actual weight. For a cruise ship, a larger tonnage figure usually means more enclosed space, not more literal tons.

It measures the ship's actual weight in tonnes, including fuel and stores. — No, this is the tempting weight reading: tonnage sounds like a scale measurement, and fuel and stores do add mass. But gross tonnage is an enclosed-volume index, while the weight of the ship and its contents is closer to displacement. That is why a quoted 250,000 gross tonnage is not the same as 250,000 tonnes on a scale.

It measures the combined weight of passengers, crew, and supplies. — No, passengers, crew, and supplies do affect loading, so this is not a random guess. But gross tonnage does not add up their body weight; it is based on the ship's enclosed volume. A crowded ship can carry more people, yet the quoted tonnage still describes space rather than a headcount on a scale.