However, when the First World War broke out, the Royal Navy requisitioned most warships under construction in Britain for foreign powers and refunded the two-thirds of the Bjørgvin's £370,000 purchase price already paid by the Norwegians.

Addition of the bulges cost 2 knots (3.7 km/h; 2.3 mph) in speed, but prevented the extra weight resulting from all of these changes from deepening her draft.

She was powered by two vertical triple expansion steam engines, which developed a total of 4,000 indicated horsepower (3,000 kW) from four Yarrow watertube boilers and gave a maximum speed of 12 knots (22 km/h; 14 mph).

She spent the next month and a half either out on patrol in the English Channel or preparing for the bombardment scheduled for the end of September in support of a major offensive along the coast.

She, and the other monitors, were attacked several times during the day by German aircraft with little effect and several coast defense batteries attempted to engage them through the smokescreen put up by the motor launches supporting the operation.

She fired thirteen shells the next day in another attempt to destroy the bridge and claimed one hit although this was not confirmed by subsequent observations.

She fired 41 rounds during the morning at a range of 26,000 yards (24,000 m), but she accompanied Vice-Admiral Keyes in the destroyer Termagant in a reconnaissance mission to see if the Germans were still holding the coast in strength.

The fire of the Tirpitz and Raversyde Batteries soon disabused them of any notions to the contrary and Gorgon was forced to turn away at maximum speed (14 knots (26 km/h; 16 mph)), which was faster than she'd made on trials, when they straddled her and hit her with splinters from the near-misses.